High-resolution magnetostratigraphy of the upper Nacimiento Formation, San Juan Basin, New Mexico, USA: implications for basin evolution and mammalian turnover

Abstract

Lower Paleocene deposits in the San Juan Basin document one of the best records of mammalian change and turnover following the Cretaceous-Paleogene extinctions and are the type section for the Puercan (Pu) and Torrejonian (To) North America Land Mammal age biozones (NALMA). One of the largest mammalian turnover events in the early Paleocene occurs between the Torrejonian 2 (To2) and Torrejonian 3 (To3) NALMA biozones. The Nacimiento Formation are the only deposits in North America where the To2-To3 mammalian turnover can be constrained, however the precise age and duration of the turnover is poorly understood due to the lack of a precise chronostratigraphic framework. We analyzed paleomagnetic samples, produced a 40Ar/39Ar detrital sanidine age, and developed a detailed lithostratigraphy for four sections of the upper Nacimiento Formation in the San Juan Basin, New Mexico (Kutz Canyon, Escavada Wash, Torreon West and East) to constrain the age and duration of the deposits and the To2-To3 turnover. The polarity stratigraphy for the four sections can be correlated to chrons C27r-C26r of the geomagnetic polarity time scale (GPTS). Using the local polarity stratigraphy for each section, we calculated a mean sediment accumulation rate and developed a precise age model, which allows us to determine the age of important late Torrejonian mammalian localities. Using the assigned ages, we estimate the To2-To3 turnover was relatively rapid and occurred over ~120 kyr (-60/+50 kyr) between 62.59 and 62.47 Ma. This rapid duration of the mammalian turnover suggests that it was driven by external forcing factors, such as environmental change driven by the progradation of the distributive fluvial system across the basin and/or changes in regional or global climate. Additionally, comparisons of the mean sediment accumulation rates between the sections that span from the basin margin to the basin center indicate that sediment accumulation rates equalized across the basin from the end of C27r through the start of C26r, suggesting an accommodation minima in the basin associated with the progradation of a distributive fluvial system into the basin. This accommodation minimum also likely led to the long hiatus of deposition between the Paleocene Nacimiento Formation and the overlying Eocene San Jose Formation.

Full text

HIGH-RESOLUTION MAGNETOSTRATIGRAPHY OF THE UPPER
NACIMIENTO FORMATION, SAN JUAN BASIN, NEW MEXICO, USA:
IMPLICATIONS FOR BASIN EVOLUTION AND
MAMMALIAN TURNOVER
CAITLIN LESLIE*
,†
, DANIEL PEPPE*, THOMAS WILLIAMSON**,
DARIO BILARDELLO***, MATTHEW HEIZLER
§
, ROSS SECORD
§§
, and
TYLER LEGGETT*
ABSTRACT. Lower Paleocene deposits in the San Juan Basin document one of the
best records of mammalian change and turnover following the Cretaceous-Paleogene
mass extinction and is the type area for the Puercan (Pu) and Torrejonian (To) North
America Land Mammal age (NALMA). One of the largest mammalian turnover events
in the early Paleocene occurs between the Torrejonian 2 (To2) and Torrejonian 3
(To3) NALMA interval zones. The Nacimiento Formation contains the only deposits in
North America where the To2-To3 mammalian turnover can be constrained; however,
the precise age and duration of the turnover is poorly understood due to the lack of a
precise chronostratigraphic framework. We analyzed paleomagnetic samples, pro-
duced a
40
Ar/
39
Ar detrital sanidine age, and developed a detailed lithostratigraphy for
four sections of the upper Nacimiento Formation in the San Juan Basin, New Mexico
(Kutz Canyon, Escavada Wash, Torreon West and East) to constrain the age and
duration of the deposits and the To2-To3 turnover. The polarity stratigraphy for the
four sections can be correlated to chrons C27r-C26r of the geomagnetic polarity time
scale (GPTS). Using the local polarity stratigraphy for each section, we calculated a
mean sediment accumulation rate and developed a precise age model, which allows us
to determine the age of important late Torrejonian mammalian localities. Using the
assigned ages, we estimate the To2-To3 turnover was relatively rapid and occurred
over ⬃120 kyr (ⴚ60/ⴙ50 kyr) between 62.59 and 62.47 Ma. This rapid duration of
mammalian turnover suggests that it was driven by external forcing factors, such as
environmental change driven by the progradation of the distributive fluvial system
across the basin and/or changes in regional or global climate. Additionally, compari-
sons of the mean sediment accumulation rates among the sections that span from the
basin margin to the basin center indicate that sediment accumulation rates equalized
across the basin from the end of C27r through the start of C26r, suggesting an
accommodation minimum in the basin associated with the progradation of a distribu-
tive fluvial system into the basin. This accommodation minimum also likely led to the
long hiatus of deposition between the Paleocene Nacimiento Formation and the
overlying Eocene San Jose Formation.
Keywords: Paleocene, paleomagnetism, rock magnetism, basin evolution, mamma-
lian evolution, North American Land Mammal age, Torrejonian
introduction
The Nacimiento Formation outcrops in the San Juan Basin of northwestern New
Mexico (fig. 1) and is composed primarily of terrestrial fluvial rocks. The Nacimiento
Formation contains one of the most continuous records of lower Paleocene terrestrial
evolution in North America and documents the early Paleocene radiation of mammals
* Terrestrial Paleoclimatology Research Group, Department of Geosciences, Baylor University, One
Bear Place #97354, Waco, Texas 76706, USA
** New Mexico Museum of Natural History and Science, Albuquerque, New Mexico 87104, USA
*** Institute for Rock Magnetism, University of Minnesota, Minneapolis, Minnesota 55455, USA
§
New Mexico Bureau of Geology, New Mexico Tech, Socorro, New Mexico 87801, USA
§§
University of Nebraska-Lincoln, Department of Earth and Atmospheric Sciences, Lincoln, Nebraska
68588, USA
†
Corresponding author: Caitlin_Leslie@baylor.edu; CaitlinELeslie@gmail.com
[American Journal of Science, Vol. 318, March, 2018,P.300–334, DOI 10.2475/03.2018.02]
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when mammals rapidly diversified following the Cretaceous-Paleogene (K-Pg) mass
extinction (Williamson, 1996). The San Juan Basin is the “type” area for the early
Paleocene Puercan (Pu) and Torrejonian (To) North American Land Mammal age
interval zones (NALMAs) that were first defined based on mammalian assemblages
from the Nacimiento Formation (Wood and others, 1941). These mammal assem-
blages continue to be critical for the characterization and definition of early Paleocene
NALMAs and their subdivisions (Archibald and others, 1987; Lofgren and others,
2004).
Matthew (1937) provided the first comprehensive review of the long history of
fossil mammalian collections from the Nacimiento Formation. Williamson (1996)
documented two significant intervals of mammalian turnover within the formation,
between Puercan 2 (Pu2) and Puercan 3 (Pu3) and between Torrejonian 2 (To2) and
Torrejonian 3 (To3) that exhibit high rates of turnover. However, the precise timing of
these turnovers and their contemporaneous nature across the basin is uncertain. The
Nacimiento Formation is the only continuous record of the To2-To3 turnover interval
in North America, therefore a more precise geochronologic framework is needed to
better evaluate biotic change through this critical interval of time.
Magnetostratigraphy has been used to develop a chronostratigraphic framework
for the Nacimiento Formation with the aim of providing a geochronologic framework
for study of vertebrate evolution and evaluating the timing of mammalian faunal
change for much of the early Paleocene (Butler and others, 1977; Taylor, ms, 1977;
Farmington
Cuba
NEW
MEXICO
50 km
N
SanJoseFormation
Nacimiento Formation
Ojo Alamo Sandstone
Fruitland & Kirtland
Formations
Pictured Cliffs
Sandstone
Ts j
Tn
To a
Kkf
Kpc
Ts j
Tn
Toa
Kkf
Kpc
Kutz Canyon
Escavada Wash
To rr e o n W e st
Torr e o n E as t
Fig. 1. Geologic map of the San Juan Basin, New Mexico showing the Cretaceous through Eocene
sediments and the locations of the four sections collected in this study, Kutz Canyon, Escavada Wash,
Torreon West, and Torreon East, indicated by white squares (modified from Williamson and others, 2008).
301C. Leslie and others
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Butler and Taylor, 1978; Lindsay and others, 1978, 1981; Taylor and Butler, 1980;
Butler and Lindsay, 1985). However, a high-resolution age model in conjunction with
detailed stratigraphy is still lacking for portions of the formation, and particularly for
the To2-To3 interval.
In this study, we focus on developing a high-resolution age model for mammalian
turnover between the To2 and To3 interval zones across the San Juan Basin. We
develop a detailed lithostratigraphy in conjunction with the Torrejonian mammal
localities and determine the magnetostratigraphy of four sections across the basin that
span the To2-To3 boundary. These sections are: Kutz Canyon, Escavada Wash, Torreon
West, and Torreon East (fig. 1). We then correlate the polarity stratigraphy of each
section to the global geomagnetic polarity time scale (GPTS) (Ogg, 2012) with the
help of a
40
Ar/
39
Ar detrital sanidine age to constrain the timing and duration of
sediment deposition in each section. Using calculated sediment accumulation rates,
we assign an age to each fossil locality within the sections to correlate the Torrejonian
mammal localities across the basin and constrain the age of the To2-To3 mammalian
turnover. Finally, we use comparisons of sediment accumulation rates among the four
sections to assess the evolution of deposition in the upper Nacimiento Formation.
previous work
Lithostratigraphy
The San Juan Basin of northwestern New Mexico is a Laramide foreland basin
containing Upper Cretaceous through lower Eocene deposits (fig. 1) (Chapin and
Cather, 1981). Cather (2004) documents three phases of subsidence in the San Juan
basin: an early phase during the late Campanian-early Maastrichtian, a medial phase
during the latest Maastrichtian-early Paleocene, and a last phase during the Eocene.
The medial phase (⬃74– 67 Ma) allowed for the deposition of the Nacimiento
Formation which is composed of alluvial deposits that are subdivided into three
members: the Arroyo Chijuillita, the Ojo Encino, and the Escavada members (William-
son and Lucas, 1992; Williamson, 1996). The Ojo Alamo Sandstone underlies the
Nacimiento Formation, representing earliest Paleocene deposition in the basin, and
the lower Eocene San Jose Formation unconformably overlies the Nacimiento Forma-
tion. This study focuses on the Ojo Encino Member of the Nacimiento Formation that
contains variegated red and drab paleosols, sheet and channel sandstone units, and
three persistent “black” paleosols that we refer to as the “lower black”, “middle black”,
and “upper black”. These black beds are useful marker beds as they can be correlated
across the basin. The abundant red beds in the Ojo Encino Member distinguish it from
the drab colored mudstone and sandstone beds of the underlying Arroyo Chijuillita
Member and the high proportion of sandstones and silcretes of the overlying Escavada
Member (Williamson and Lucas, 1992; Williamson, 1996; Davis and others, 2016).
The four sections investigated in this study are spread along a northwest-southeast
transect through the basin and span from basin center to basin margin (fig. 1). Kutz
Canyon, the northernmost section, is near the center of the basin, Escavada Wash is
near the basin margin approximately 48 km southeast of Kutz Canyon, and Torreon
West and East, the southernmost sections, are approximately 32 km southeast of
Escavada Wash near the basin margin. Torreon West and East are separated by about
10 km. Taylor (ms, 1977) produced the first stratigraphic sections, collected paleomag-
netic samples, and documented the stratigraphic position of fossil mammal localities
for Torreon West, Torreon East, and the Big Pocket fossil locality in Kutz Canyon. In
the measured sections, lithologies were generalized and lumped into siltstone, sand-
stone, volcanic ash, and coal intervals. Taylor (ms, 1977) also identified lower, middle,
and upper “carbonaceous” clay beds that outcrop as distinct black intervals, referred to
the lower, middle, and upper blacks in this paper. Williamson (1996) subsequently
302 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
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produced more detailed measured sections for Torreon West, Torreon East, Kutz
Canyon, and Escavada Wash by documenting pedogenic features and reporting
Munsell colors for mudstones. However, his sections are not tied directly to magneto-
stratigraphic sections, and thus are not well constrained geochronologically.
Paleomagnetism and Rock Magnetism
Taylor and Butler (1980) constructed the magnetic polarity stratigraphy of a
185 m section at Torreon West, a 62 m section at Torreon East, and a 420 m section at
Kutz Canyon. The Deltatherium mammalian zone (equivalent to Torrejonian 2, sensu
Lofgren and others, 2004) was recognized at all three sections and the “Pantolambda
mammalian zone” (sensu Osborn, 1929, see below; equivalent to Torrejonian 3, sensu
Lofgren and others, 2004) at Torreon West and Kutz Canyon. The two zones were
estimated to be separated by about 500 kyr. The section at Kutz Canyon, measured at
the Big Pocket fossil locality, includes approximately 100 m of strata below the
Deltatherium zone. The paleomagnetic sample spacing for all sections was approxi-
mately 3 meters. The Torreon West, Torreon East, and upper portion of the Kutz
Canyon sections were correlated to C26r to C25r based on previous work correlating
the Upper Cretaceous through middle Paleocene San Juan Basin deposits to the
geomagnetic polarity time scale (GPTS) (Taylor, ms, 1977; Lindsay and others, 1978).
Following the work of Taylor and Butler (1980), Butler and Lindsay (1985) recognized
magnetic overprinting in the lower part of the section and revised correlation of the
polarity zones to C27r to C26r rather than C26r to C25r. Despite the numerous
mammalian localities identified at Escavada Wash, a polarity stratigraphy has not been
constructed until now.
Taylor and Butler (1980) identified the persistent black mudstones at Torreon
West and East that function as excellent marker beds to reference the stratigraphic
position of reversals in each section. The C27r-C27n reversal was reported near the
base of the middle black at Torreon West and near the top of the middle black at
Torreon East and the C26r-C27n reversal was reported at the base of the upper black at
Torreon West and 9 m below the base at Torreon East. These findings suggest that the
blacks are diachronous despite the proximity of the two sections.
A rock magnetic study of the San Juan Basin deposits was also conducted by Butler
and Lindsay (1985) who analyzed magnetic separates from nine stratigraphic levels
through Upper Cretaceous to middle Paleocene deposits; however, the exact sample
locations are unclear and therefore it is unknown whether any sample overlap exists
with the new sections presented here. Butler and Lindsay (1985) determined that
titanohematite of intermediate composition was the dominant magnetic mineral
based on microprobe and X-ray analyses, supported by Curie temperatures ranging
from 180 to 300 °C. These findings indicate the sediments were likely sourced from
volcanic rocks of the San Juan Mountain region to the north, based on the presence of
titanohematite. Kodama (1997) measured the anisotropy of remanence on samples of
the Nacimiento Formation collected from Kutz Canyon to conduct an inclination
shallowing correction and found support for a primary detrital magnetization for the
formation.
Mammalian Biostratigraphy
Mammalian interval zones in the San Juan Basin have a long and complicated
history. Sinclair and Granger (1914) established the presence of two zones of distinct
mammalian assemblages at Torreon Wash, the Deltatherium and Pantolambda zones,
which are separated by approximately 30 m of non-fossiliferous deposits. Osborn
(1929) treated the two zones as separate life zones representing different ages and
formally named and characterized the Deltatherium zone by the presence of Deltatherium
fundaminis,Mioclaenus turgidus, and Haploconus angustus and the Pantolambda zone by
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the presence of Pantolambda cavirictum and Arctocyon ferox. However, others considered
these two zones to be a facies effect, sampling bias, or inaccurate due to the presence of
the same species in both zones (Granger, 1917; Matthew, 1937; Wilson, 1956; Russell,
1967). Using magnetostratigraphy, Taylor and Butler (1980) demonstrated that the
Deltatherium and Pantolambda zones represent different ages at Torreon Wash. Taylor
(ms, 1984) renamed the Deltatherium and Pantolambda zones Deltatherium – Tetraclaeno-
don (D-T) and Pantolambda bathmodon –Mixodectes pungens (P-M) after the discovery of
Pantolambda cavirictum in other localities in the basin within the upper portion of the
“Deltatherium zone”.
Archibald and others (1987) updated the Paleocene NALMAs and divided the
Torrejonian into three zones, To1-To3, using the San Juan Basin mammalian assem-
blages to establish the To2 and To3 mammal ages. The Deltatherium and Pantolambda
zones established by Sinclair and Granger (1914) are equivalent to To2 and To3,
respectively, of Archibald and others (1987). Archibald and others (1987) defined To2
as the Tetraclaenodon-Pantolambda zone and To3 as the Pantolambda-Plesiadapis praecursor
zone, placing the boundary between the two zones within the upper part of the
Deltatherium zone of Sinclair and Granger (1914). Williamson (1996) further subdi-
vided the To2 and To3 interval into five biostratigraphic zones within the San Juan
Basin: P-E zone (lowest occurrence of Protoselene opisthacus and lowest occurrence of
Ellipsodon granger), E-A zone (lowest occurrence of Ellipsodon granger and first occur-
rence of Arctocyon ferox), A-P zone (lowest occurrence of Arctocyon ferox and lowest
occurrence of Pantolambda cavirictum), P-M zone (lowest occurrence of Pantolambda
cavirictum and first occurrence of Mixodectes pungens), and M zone (first and last
occurrence of Mixodectes pungens). Williamson (1996) defined the A-P zone exclusively
from Kutz Canyon and the P-M zone from Torreon West, Kutz Canyon, and two other
locations in the basin, Kimbeto and Betonnie-Tsosie washes. The M zone was defined
from Torreon East and West and Escavada Wash.
In their revision of Paleocene NALMAs, Lofgren and others (2004) redefined the
To2 interval zone as the Protoselene opisthacus-Mixodectes pungens interval zone, based
primarily on the faunal succession observed at Kutz Canyon and the To3 interval zone
as the Mixodectes pungens-Plesiadapis praecursor interval zone based primarily on the
faunal record at Torreon Wash. With these revisions, To2 is approximately equivalent
to the duration of the local San Juan Basin biozones P-E through P-M (Williamson,
1996), and approximately equivalent to the Deltatherium zone (Sinclair and Granger,
1914). To3 is nearly equivalent to the duration of the M biozone (Williamson, 1996)
and the Pantolambda zone (sensu Sinclair and Granger, 1914).
Using the Torrejonian zonation of Lofgren and others (2004), there are signifi-
cant changes in therian mammals across the To2-To3 boundary in the San Juan Basin.
The changes observed between the last two San Juan Basin biozones in To2, the A-P
and P-M zones, and To3, the M zone are locally considerable (Williamson, 1996).
Intense collecting efforts over the last twenty years have resulted in modifications of
the San Juan Basin biozones of Williamson (1996), including the movement of several
localities from the P-M zone to the older A-P zone (table 1). Despite this extensive
collecting, collection biases remain among the fossil zones. For example, the A-P and
P-M zones contain few microvertebrate sites, but several are present in the M zone.
Additionally, the A-P and P-M zones are generally less fossiliferous and have signifi-
cantly fewer fossil sites than the M zone.
Despite potential issues with collection biases, there are notable differences
between the P-M and M zones in species occurrences and abundance. Several taxa
found in the P-M zone, but not the M zone represent medium- to large-bodied taxa
that are abundant in P-M zone faunas, making their disappearance across the To2-To3
boundary particularly significant. These are the ‘triisodontid’ ‘condylarth’ Triisodon
304 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
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TABLE 1
Selected Mammal Localities and Calculated Ages
Mammal Localities
NALMA
interval zone
(Lofgren and
others, 2004)
San Juan
Basin biozone
(Williamson,
1996)
Strati-
graphic
position
(m)
Calculated age (Ma)
Kutz Canyon
Sedimentation rate: 119.9 (-16.3, +18.7) m/myr
1: L-00400 To2 A-P zone 1-6 62.76-62.71 (-0.04, +0.05)
2: L-06419 To2 A-P zone 1-6 62.76-62.71 (-0.04, +0.05)
3: L-09907, L-09908, L-09909 To2 A-P zone 10 62.68 (-0.00, +0.00)
4: L-08234 To2 P-M zone 16 62.63 (-0.00, +0.01)
Escavada Wash
Sedimentation rate: 115.7 (-30.3, +34.7) m/myr
5: L-10453 To2 P-M zone 6.5 62.80 (-0.09, +0.11)
6: L-09187 To2 P-M zone 15.25 62.72 (-0.08, +0.08)
7: L-09985 To2 P-M zone 15.25 62.72 (-0.08, +0.08)
8: L-10554 To2 P-M zone 15.25 62.72 (-0.08, +0.08)
9: L-10350 To2 P-M zone 16.5 62.71 (-0.07, +0.08)
10: L-09981 To3 M zone 59 62.35 (-0.10, +0.05)
11: L-10444 To3 M zone 59 62.35 (-0.10, +0.05)
12: L-10556 To3 M zone 61 62.33 (-0.10, +0.06)
13: L-09982 To3 M zone 62 62.32 (-0.10, +0.06)
14: L-10415 To3 M zone 63 62.31 (-0.11, +0.06)
15: L-10660 To3 M zone 63.25 62.31 (-0.11, +0.06)
16: L-10414 To3 M zone 64.5 62.30 (-0.11, +0.06)
17: L-10403 To3 M zone 65 62.29 (-0.11, +0.06)
18: L-09197 To3 M zone 65.15 62.29 (-0.11, +0.06)
Torreon West
Sedimentation rate: 119.9 (-10.1, +11.6) m/myr
19: L-08182 To2 A-P zone 19.75 62.66 (-0.03, +0.04)
20: L-08178 To2 A-P zone 23.25 62.63 (-0.03, +0.03)
21: L-08180 To2 P-M zone 27.5 62.59 (-0.02, +0.03)
22: L-09173 To3 M zone 43 62.46 (-0.02, +0.03)
23: L-08183 To3 M zone 45 62.45 (-0.02, +0.03)
24: L-10500 To3 M zone 48 62.42 (-0.03, +0.03)
25: L-06898 To3 M zone 50.5 62.40 (-0.03, +0.03)
26: L-10490 To3 M zone 55.5 62.36 (-0.03, +0.04)
27: L-07108 To3 M zone 57 62.35 (-0.03, +0.04)
28: L-07582 To3 M zone 57 62.35 (-0.03, +0.04)
29: L-10493 To3 M zone 57.75 62.34 (-0.03, +0.04)
30: L-10494 To3 M zone 57.75 62.34 (-0.03, +0.04)
31: L-10495 To3 M zone 57.75 62.34 (-0.03, +0.04)
32: L-01121 To3 M zone 58.5 62.33 (-0.03, +0.04)
33: L-10534 To3 M zone 59.25 62.33 (-0.04, +0.04)
34: L-10558 To3 M zone 59.25 62.33 (-0.04, +0.04)
35: L-10561 To3 M zone 59.25 62.33 (-0.04, +0.04)
36: L-07848 To3 M zone 59.75 62.32 (-0.04, +0.04)
37: L-08205 To3 M zone 59.75 62.32 (-0.04, +0.04)
38: L-10506 To3 M zone 61.5 62.31 (-0.04, +0.04)
39: L-10535 To3 M zone 61.5 62.31 (-0.04, +0.04)
40: L-10512 To3 M zone 63.5 62.29 (-0.04, +0.04)
41: L-10560 To3 M zone 63.5 62.29 (-0.04, +0.04)
42: L-10505 To3 M zone 66 62.27 (-0.04, +0.04)
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quivirensis, the periptychid ‘condylarth’ Haploconus angustus, and the enigmatic ‘condy-
larth’? Deltatherium fundaminis. The taeniodont Huerfanodon torrejonius and the mio-
claenid ‘condylarth’ Ellipsodon yotankae are uncommon in the P-M zone and do not
appear in the M zone. Considering the difference in collection size and localities
between the P-M and M zones, the disappearance of these taxa appears to be
significant. Several taxa reported from the M zone, but not from the P-M zone (Picrodus
calgariensis, the palaeanodont? Escavadodon zygus, the hyopsodontid ‘condylarth’ Hap-
laletes sp., and the arctocyonid ‘condylarth’ Colpoclaneus procyonoides) are rare in the M
zone, and thus their absence from the P-M zone could be due to sampling biases
between the zones.
In addition to differences in taxonomic occurrences, there are notable differ-
ences in the morphology of the mammalian taxa between the P-M and M zones.
Several of the changes in mammal taxa between the two zones may reflect anagenic
changes within a lineage as they represent two species, usually included within a single
genus, distinguished primarily by a significant difference in size, for example: Swaindel-
phys johansoni and S. encinensis,Mixodectes malaris and M. pungens (contra Tsentas, 1981
and Szalay and Lucas, 1996, M. malaris is not present in the M. zone), Anasazia
williamsoni and Torrejonia wilsoni, and Pentacodon inversus and P. occultus. Interestingly,
for all these “species pairs”, the M zone taxon is larger than the P-M zone taxon.
Although the mammalian turnover between To2 and To3 is well defined, the
fossils come from different locations and the exact timing of the To2-To3 turnover is
uncertain. This demonstrates the need for a robust age model for the To2-To3
intervals in the Nacimiento Formation to allow for correlations of the San Juan Basin
mammalian assemblages to other contemporaneous assemblages across North Amer-
ica and to other continents, and to precisely constrain the age of the To2-To3 turnover.
For the purposes of this paper, we use the definition of Lofgren and others (2004) for
the To2 and To3 interval zones.
TABLE 1
(continued)
Mammal Localities
NALMA
interval zone
(Lofgren and
others, 2004)
San Juan
Basin biozone
(Williamson,
1996)
Strati-
graphic
position
(m)
Calculated age (Ma)
Torreon East
Sedimentation rate: 87.0 (-8.0, +9.0) m/myr
43: L-07725 To2 A-P zone 18.5 62.66 (-0.03, +0.04)
44: L-08227 To2 A-P zone 18.5 62.66 (-0.03, +0.04)
45: L-08228 To2 A-P zone 18.5 62.66 (-0.03, +0.04)
46: L-07583 To2 A-P zone 21.25 62.63 (-0.02, +0.04)
47: L-04954 To2 A-P zone 21.25 62.63 (-0.02, +0.04)
48: L-04950 To2 A-P zone 21.25 62.63 (-0.02, +0.04)
49: L-10013 To3 M zone 35 62.47 (-0.02, +0.03)
50: L-10432 To3 M zone 35 62.47 (-0.02, +0.03)
51: L-09166 To3 M zone 39 62.43 (-0.02, +0.03)
52: L-09167 To3 M zone 39 62.43 (-0.02, +0.03)
53: L-01079 To3 M zone 52 62.28 (-0.04, +0.05)
54: L-09169 To3 M zone 52 62.28 (-0.04, +0.05)
55: L-10454 To3 M zone 52 62.28 (-0.04, +0.05)
56: L-09172 To3 M zone 52 62.28 (-0.04, +0.05)
306 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
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methodology
Measured Sections
A total of four sections were measured: a 70 m section at Kutz Canyon, a 87 m
section at Escavada Wash, a 79 m section at Torreon West, and a 76 m section at
Torreon East (figs. 2 and 3). At Torreon West and Torreon East, the base of the
lithostratigraphic and magnetostratigraphic sections was the lower black marker bed.
The Escavada Wash section contains multiple middle black marker beds so the section
began at the lowest black marker bed present at that locality. Kutz Canyon does not
contain the lower or middle black marker beds, so this section was measured from a
known late To2 mammal locality (Bab’s Basin) to ensure overlap with the other
sections.
At each section, the outcrop was trenched to remove weathered material and
document the lithologic contacts. The paleosols were described at the centimeter
scale. The texture, color, ped structure, and shrink-swell features of each paleosol were
documented using the guidelines of Soil Survey Staff (1999). For sandstones, the grain
size and relationship to underlying strata was characterized.
The stratigraphic positions of mammal localities that occur within each measured
section were documented in the field (figs. 2 and 3). Some mammal localities occur
lateral to the measured sections and these localities were placed into each section
using measurements from marker beds. Here we recognize A-P zone localities at
Torreon West and Torreon East for the first time. Table 1 shows each selected mammal
locality (localities that contain multiple taxa and can be precisely placed into the
section), the numeric code it has been given for this paper, the associated NALMA
interval zone, San Juan Basin biozone, and stratigraphic position.
Paleomagnetism
Four paleomagnetic block samples were collected from paleosols, mudstone, and
fine grained sandstone beds at ⬃1 m intervals (0.25 m minimum and 7 m maximum
sample spacing) in each section, defining a study site. Lithologies coarser than fine
grained sandstones were avoided. Site spacing was primarily dictated by stratigraphic
exposure and lithology. When sampling, a flat face was shaved onto the samples in situ
using a hand rasp and its orientation was measured using a Brunton Pocket Transit
Compass. In the laboratory, the samples were cut into approximately 4 cm
3
cubes using
a diamond-bit saw. Each sample yielded one cubic specimen.
At Kutz Canyon and Escavada Wash, the entire sections spanning from late To2
through To3 strata were sampled. A total of 48 sites were sampled at Kutz Canyon and
50 sites at Escavada Wash. At Torreon West and East, the position of reversals had been
reported by Taylor and Butler (1980) for the entire late To2-To3 succession. Conse-
quently, full sections were not sampled but rather the deposits surrounding the
approximate reversal positions were densely sampled to better constrain each reversal.
A total of 38 sites were sampled at Torreon West and 44 sites at Torreon East.
Specimens were measured at Baylor University using a 2G Enterprises (Mountain
View, California) cryogenic DC-SQuID magnetometer located in a 2-layer magneto-
static shielded room with a background field typically less than 300 nT. All specimens
were demagnetized using a thermal demagnetization strategy. Rock magnetic analyses
determined the magnetic carriers in the samples had relatively low unblocking
temperatures (see below), therefore thermal demagnetization steps were performed
in 25° increments up to the maximum unblocking temperature or until the magnetiza-
tions became erratic and unstable, ranging between 225° to 400 °C. To minimize
oxidation reactions, thermal demagnetization was performed using a nitrogen atmo-
sphere using ASC (Carlsbad, California) controlled atmosphere thermal demagne-
tizer.
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Principal-component analysis (PCA) was used to isolate the characteristic rema-
nence for each demagnetized specimen (Kirschvink, 1980). A best-fit line was calcu-
lated from at least three demagnetization steps that trended toward the origin and had
16
0
10
20
30
40
50
60
70
m
80
M MM U
-90 0 90
VGP latitude
=linefitwithMAD<20°
= site mean
U
1,2
3
msvf f mcvc
A-
C-
B+
L
4
A-
C-
B+
5
6,7,8
9
10,11
12 13
14
15
17
18
Kutz Canyon Escavada Wash
msvf f mcvc
-90 090
VGP latitude
= line fit with MAD <20°
= site mean
0
m
10
20
30
40
50
60
70
M
C26rC27r C27n
= detrital sanidine age
Base: 13S, 241518.6 m E, 4048605.3 m N
Top: 13S, 242771.9 m E, 4049052.9 m N
Base: 13S, 264743.9 m E, 4006639.6 m N
Top: 13S, 265545.7 m E, 4008764.9 m N
Fig. 2. Measured sections with fossil localities, VGP latitude, and interpreted polarity zonations for the
Kutz Canyon and Escavada Wash sections. Note the presence of the black beds highlighted and labeled L
(lower), M (middle) and U (upper). The base of the B⫹magnetozone is used as the datum. Fossil localities
1–4 from Kutz Canyon and 5–18 from Escavada Wash are described in table 1. Section base and top UTM
coordinates shown, NAD27 datum.
308 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
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a maximum angle of deviation (MAD) ⬍20° (Appendix 1). Data from specimens that
had erratic demagnetization behaviors were excluded from further analysis. A site
mean direction was calculated for all sites with three specimens with statistically
significant directions using Fisher statistics (Appendix 2) (Fisher, 1953). If site means
had a 95 percent confidence circle (␣
95
)⬎35°, which exceeds cut-off values based on
m
U
LM
msvf f mcvc
19
20
21
22
23
24
25
26
40,41
42
27,28
29,30,31
32
33,34,35
38,39
36,37
m
M
LU
msvf f mcvc
43,44,45
46,47,
48
49,50
51,52
53,54,
55,56
C26r
C27r C27n
Torreon West Torreon East
B+
B+
C-
C-
A- A-
-90 0 90
VGP latitude
= line fit with MAD <20°
= site mean
-90 0 90
VGP latitude
=linefitwithMAD<20°
=sitemean
10
0
20
30
40
50
60
70
0
10
20
30
40
50
60
70
Base: 13S, 294487.6 m E, 3985480.9 m N
Top: 13S, 294885.9 m E, 3986124.9 m N
Base: 13S, 285714.0 m E, 3988696.0 m N
Top: 13S, 286383.0 m E, 3989637.9 m N
Fig. 3. Measured sections with fossil localities, VGP latitude, and interpreted polarity zonations for the
Torreon West and Torreon East sections. Note the presence of the black beds highlighted and labeled L
(lower), M (middle) and U (upper). The base of the B⫹magnetozone is used as the datum. Fossil localities
19– 42 from Torreon West and 43–56 from Torreon East are described in table 1.
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the randomness criteria of Watson (1956), they were not used. Reversal boundaries
were placed at the stratigraphic midpoints between samples of opposing polarity. The
local polarity stratigraphy was then correlated to the geomagnetic polarity time scale
(GPTS) (Ogg, 2012). The polarity stratigraphy of the Torreon West and East deposits
coarsely resolved by Taylor and Butler (1980) determined the base and top of each
section was reversed, therefore the deposits below and above the sampling intervals in
this study were interpreted to be reversed.
Rock Magnetism
Rock magnetic analyses were performed at the Institute for Rock Magnetism at the
University of Minnesota. To characterize the full suite of magnetic mineralogies
present within the study interval, representative samples were chosen that captured the
range of lithologies present. The lithologies were categorized into the following
categories: black slickensided paleosols, red calcareous paleosols, brown to gray
slickensided paleosols, drab-colored slickensided paleosols, and silty weakly-developed
paleosols. Saturation magnetization was measured on a MicroMag Princeton Measure-
ments Corporation (Princeton, New Jersey) vibrating sample magnetometer (VSM) on
11 specimens at temperatures ranging from 30°C to 600 °C.
A Quantum Design (San Diego, California) Magnetic Properties Measurement
System (MPMS) was used for low-temperature remanence measurements on 18
specimens. The protocol included field-cooled (FC) remanence, zero-field-cooled
(ZFC) low-temperature saturation isothermal remanent magnetization (LTSIRM),
and room temperature saturation isothermal remanent magnetization (RTSIRM).
The method involves applying a sustained DC field of 2.5 T as a specimen is cooled
from 300 K to 20 K (FC) and its magnetic remanence is measured upon warming back
to room temperature (300 K). Subsequently the specimen is cooled to 20 K with no
applied field (ZFC) and a 2.5 T LTSIRM is applied. The magnetic remanence is
measured while the specimen warms back to 300 K. A 2.5 T room temperature SIRM
(RTSIRM) is then applied at 300K and the magnetic remanence is measured upon
cooling to 20 K and warming back to room temperature.
An additional goethite test was performed on four specimens to determine
whether goethite was present. The method, similar to that of Guyodo and others
(2006), involves heating a sample up to 400 K and then applying a thermal remnant
magnetization (TRM) as the sample cools from 400 to 300 K, magnetizing any goethite
through its Ne´el temperature. The applied field is then turned off and the remanence
is measured as the specimen is cooled to 20 K and warmed back up to 300 K, allowing the
specimens to cycle through the magnetite structural phase transition temperature of ⬃120
K (the Verwey transition, for example Dunlop and Ozdemir, 1997) resulting in loss of
remanence if any magnetite is present. The specimen is then heated to 400 K again in zero
field, demagnetizing goethite through its Ne´el ordering temperature. Remanence is
measured while the sample cools to 20 K and warms to 300 K to determine which, if any,
magnetic phases remain after the contribution of goethite is removed.
A triaxial-IRM Lowrie test was performed at Baylor University on 11 samples to
determine the primary magnetic carrier on mixed mineralogy samples (Lowrie, 1990).
11 cubic specimens were imparted with a 1T, 300 mT, and 100 mT field along the X, Y,
and Z axes respectively with an ASC pulse magnetizer. Samples were thermally
demagnetized in 50 °C increments from 100 to 650 °C in an ASC controlled atmo-
sphere thermal demagnetizer. Samples were measured using the 2G cryogenic DC-
SQuID magnetometer.
Detrital Sanidine Dating
One sample, H16-SJ-11, was collected from a sandstone body at 64 m in Escavada
Wash (fig. 2). Processing and mineral separation was done at the New Mexico
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Geochronology Research Laboratory (NMGRL). Sample separation included: crush-
ing & grinding the rock sample, cleaning with H
2
O and 5 percent HF acid, and sieving
grain size between 40 and 80 mesh. Magnetic and heavy liquid density separation was
used to concentrate the K-feldspar grains. Sanidine was hand-picked from the bulk
K-feldspar separate based on optical properties indicative of sanidine as viewed while
immersed in wintergreen oil under a polarizing binocular microscope. Approximately
100 grains were selected and then washed in acetone to remove the wintergreen oil.
The crystals were irradiated at the TRIGA reactor in Denver, Colorado, for 8 hours
in the NM-291 package along with Fish Canyon Sanidine interlaboratory standard FC-2
with an assigned age of 28.201 Ma (Kuiper and others, 2008). Ages are calculated with
a total
40
K decay constant of 5.463e-10 /a (Appendix 3) (Min and others, 2000).
After irradiation, six crystals of FC-2 from each of 8 monitor holes from a 24-hole
irradiation tray, along with 89 sample crystals were loaded into a copper tray, evacuated
and baked at 140 °C for 2 hours. Sample crystals were fused with a CO
2
laser and the
extracted gas was cleaned with a NP 10 getter operated at 1.6 A for 30 seconds. The gas
was analyzed for argon isotopes using an ARGUS VI multicollector mass spectrometer
equipped with five Faraday cups, and one ion counting multiplier (CDD). The
configuration had
40
Ar,
39
Ar,
38
Ar,
37
Ar and
36
Ar on the H1, AX, L1, L2, and CDD
detectors, respectively. H1, AX and L2 utilized Faraday detectors equipped with
1⫻10
13
Ohm resistors whereas L1 had a Faraday with a 1⫻10
14
Ohm resistor. The CDD
is an ion counting detector with a dead time of 14 nS. All data acquisition was
accomplished with NM Tech Pychron software and data reduction used Mass Spec (v.
7.875) written by Al Deino at the Berkeley Geochronology Laboratory. Extraction line
blank plus mass spectrometer background values are averages of numerous measure-
ments interspersed with the unknown measurements. These values are 3⫾12 percent,
0.16⫾12 percent, 0.05⫾11 percent, 0.3⫾4 percent, 0.03⫾2.5 percent, ⫻10
⫺17
moles
from masses 40, 39, 38, 37, and 36, respectively.
The minimum age population is defined by choosing the youngest dates that form
a near Gaussian distribution as defined by the MSWD value of the distribution. The
minimum age is the inverse variance weighted mean of the selected crystals and the
error is the square root of the sum of 1/␴
2
values. The error is also multiplied by
the square root of the MSWD for MSWD greater than 1. J-error (0.03%) is included for
the weighted mean age error and all errors are reported at 1␴.
results
Magnetostratigraphy
One hundred and forty-four samples were analyzed from 48 sampling horizons
from Kutz Canyon, 150 samples from 50 sampling horizons from Escavada Wash, 114
samples from 38 sampling horizons from Torreon West, and 132 samples from 44
sampling horizons from Torreon East. The demagnetization trajectory of most speci-
mens trended towards the origin after a few steps and were fully demagnetized by 200°
to 400 °C (figs. 4A, 4B, and 4C). All specimens with coherent demagnetization
trajectories and stable endpoints were characterized by line fits. In total, reliable
paleomagnetic directions were obtained for 353 specimens from 160 sampling hori-
zons (fig. 5A; Appendix 1): 96 specimens (27% of total specimens) from 46 sampling
horizons from Kutz Canyon, 114 specimens (32% of total specimens) from 47
sampling horizons from Escavada Wash, 53 specimens (15% of total specimens) from
31 sampling horizons from Torreon West, and 90 specimens (25% of total specimens)
from 36 sampling horizons from Torreon East.
Of these sampling horizons, sixty-six (41% of total sampling horizons) had at least
three samples with statistically significant directions that could be used to calculate
site-mean directions with ␣
95
s⬍35°: 15 from Kutz Canyon, 26 from Escavada Wash, 8
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from Torreon West, and 17 from Torreon East (fig. 5B; Appendix 2). The remaining
187 samples (35% of the data) had erratic demagnetization trajectories and coherent
directions could not be obtained (fig. 4D).
The site-mean directions were averaged by section according to their polarity (fig.
5C), and also averaged at the formation-level, yielding normal (chron 27n) and
reversed (chrons 26r and 27r) Upper Nacimiento Formation-mean directions (fig.
5D), and were used to calculate mean VGP latitude and longitude (table 2). The
average Upper Nacimiento Formation direction for all the normal site-mean direc-
tions is oriented 352.8°, 56.1° (␣
95
⫽4.9°, n⫽28), whereas that of all the reversed
polarity site-mean directions is oriented 164.4°, ⫺55° (␣
95
⫽5°, n ⫽38). The reversal
test of McFadden and McElhinny (1990) returned a positive, class A test, indicating
that these directions share a common mean at the 95 percent confidence level. These
directions plot close to the expected Paleocene (62.5 Ma) direction of 341.8°, 60.1°
recalculated from Torsvik and others (2008) and our mean sampling location. In
addition, these directions also agree with the mean characteristic remanent direction
of 342.1°, 49.6° (␣
95
⫽7.1°, n ⫽20) for the Nacimiento Formation reported by
Kodama (1997) after reversing all site-mean directions to normal polarity.
The mean VGP latitude and longitude calculated from all reversed (C26r and
C27r) polarity sites is 77.2°N, 171.0°E (n⫽38; A
95
⫽6.2), and that calculated from the
N
E
W
N
E
W
N
E
W
BA
CD
N
W
S
E
West
North, up
West
N
West
North, up
West
SS
S
C27r
P16EM13A
C27n
P16EM33C
C26r
C15KC48A C15ET01D
NRM 300
300
325
400
NRM
100
325
100
275 250
225
North, up
175
NRM
NRM
100
100
125
175
300
275
150
150
225
200
325
325
400
NRM
NRM
100
125
150
225
250
275
300
250
275
225
125
150
350 225
NRM
100
125
150
175
200
225
250
275
300
325
NRM
100
125
150
175
200
275
400
325
NRM
275
325
125
175
NRM
125
150
250
275
300
325
250
NRM
125
175
275
150
350
375
400
350
400
400
400
350
375
400
375
400
350
375
325
Expected direction Antipodeof expected direction Temperaturestep not used Present dipole field
Fig. 4. Representative orthogonal end vector demagnetization diagrams and equal-area plots for each
subset of data. (A) Demagnetization trajectory of a reversed polarity sample from C27r that allowed
line-fitting to determine a characteristic direction (21% of data). (B) Demagnetization trajectory of a normal
polarity sample from C27n where a line was calculated (27% of data). (C) Demagnetization trajectory of a
reversed polarity sample from C26r where a line was calculated (17% of data). (D) Representative sample of
erratic data that was not used for interpretations (35% of data).
312 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
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normal polarity sites (C27n) is 84.1°N, 184.3°E (n⫽28; A
95
⫽6.1), and can be
compared to the expected paleopole of 74.9°N, 7.6°E recalculated from Torsvik and
others (2008).
Figures 2 and 3 show the local polarity stratigraphy for each section including
each specimen and site mean polarity and VGP latitude. At Kutz Canyon, the lower
reversal is constrained withina1minterval while the upper reversal is constrained
within a 4.5 m interval. Lower resolution on the latter is due to the presence of coarse
A
W
N
E
N
B
S
E
D
W
S
C
EW
EW
KC
TE
TE
TW
TW
KC
Lower hemisphere
Upper hemisphere
Present dipole field
Lower hemisphere expected direction
Antipode to expected direction
Kodama (1997) Nacimiento mean direction
Fig. 5. (A) Equal-area plot of all characteristic magnetization directions obtained from this study. (B)
Equal-area plot of normal and reversed site-mean directions from all sections. (C) Equal-area plot of normal
and reversed site-mean directions averaged by section, and plotted alongside the present-day field position,
the early Paleocene direction recalculated from Torsvik and others (2008) (see text for details), and the
antipode to the early Paleocene direction. The ellipse around the mean direction represents the 95%
confidence cone (Fisher, 1953). TW: Torreon West, TE: Torreon East, KC: Kutz Canyon, EW: Escavada
Wash. (D) Upper Nacimiento Formation normal and reversed formation-mean directions plotted alongside
the present-day field position, the early Paleocene direction recalculated from Torsvik and others (2008),
the antipode to the early Paleocene direction, and the mean Nacimiento Formation direction reported by
Kodama (1997). The ellipse around the mean direction represents the 95% confidence cone (Fisher, 1953).
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grained sandstone at the top of the section that was not sampled (fig. 2; table 3). The
presence of coarse grained sandstones and friable paleosols near the reversals at
Escavada Wash resulted in poorer resolution. The lower reversal is constrained within
an 8 m interval and the upper reversal is constrained within a 6.5 m interval (fig. 2;
table 3). At Torreon West, the lower and upper reversals occur within 1.2 m and 0.4 m
intervals, respectively (fig. 3; table 3). We assumed that the underlying deposits were
reversed, based on the results of Taylor and Butler (1980). The lower and upper
reversals at Torreon East are constrained within 0.5 m and 1 m intervals, and the
underlying deposits were assumed to be reversed (fig. 3; table 3).
Rock Magnetism
Rock magnetic analyses show that the upper Nacimiento Formation has a mixed
magnetic mineralogy. Low temperature magnetometry for a representative sample of a
black slickensided paleosol shows a two-fold increase on cooling of the RTSIRM and
the magnetite Verwey transition (⬃120 K), indicating that goethite and magnetite
coexist in the same specimen (for example, figs. 6A and 6B).
High temperature susceptibility curves for representative red calcareous paleosol
specimens decreased until ⬃125 °C and subsequently dropped at ⬃550 °C, suggesting
the presence of goethite and titanomagnetite (figs. 6C and 6I). Low temperature
magnetic measurements for these specimens also reveal a hematite Morin transition
(⬃260 K) and a diffuse Verwey transition between ⬃130 to 90 K, indicating the
presence of magnetite in different oxidation states (maghemite).
Low temperature magnetometry measurements from a brown, weakly-developed
paleosol specimen reveal a magnetite Verwey transition and a ⬃two-fold increase of
magnetic remanence upon cooling, indicating the presence of goethite (fig. 6E). A
goethite test performed on the same specimen (fig. 6F) shows that remanence is lost
when warming to 400 K as goethite is demagnetized through its Ne´el ordering
TABLE 2
Mean Paleomagnetic Directional Data From the Upper Nacaimiento Formation
Subset nD (°) I (°) k a95 (°) Pole (°N) Pole (°E) K A95 (°)
Kutz Canyon normal sites 9359.3 55.2 57.0 5.3 89.6 203.6 47.3 7.6
Kutz Canyon reversed sites 6169.5 -53.7 12.3 19.9 79.5 174.8 9.3 23.1
Escavada Wash normal
sites
13 349.7 54.6 20.1 9.5 81.9 174.4 12.2 12.4
Escavada Wash reversed
sites
13 164.3 -55.3 30.4 7.6 77.6 168.3 18.6 9.9
Torreon West normal sites 5351.2 58.8 116.3 7.1 82.1 195.1 83.0 8.4
Torreon West reversed
sites
3168.4 -54.2 38.1 20.3 81.5 162.0 28.6 23.5
Torreon East normal site 1337.3 66.5 -- 9.3 -- -- -- --
Torreon East reversed sites 16 161.6 -55.4 19.2 8.6 75.1 172.8 12.5 10.9
C27r- sites 16 166.7 -53.2 24.8 7.6 79.0 161.5 19.3 8.6
C27n - sites 28 352.8 56.1 32.5 4.9 84.1 184.3 20.8 6.1
C26r - sites 22 162.5 -56.3 20.4 7.0 75.6 178.5 12.6 9.1
C26r + C27r - sites 38 164.4 -55.0 22.3 5.0 77.2 171.0 14.9 6.2
Note: n – number of site-mean directions used to calculate the section and formation averages (see text for
details); D – declination; I – inclination; k – Fisher’s (1953) precision parameter; a95 – radius of 95% confidence cone
around mean (Fisher, 1953); pole N and E – mean of virtual geomagnetic poles calculated from each line or site mean;
K and A95 – Fisher statistics of paleomagnetic pole.
314 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
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TABLE 3
Stratigraphic Position of Polarity Zone Boundary
Local polarity zone Chron Location Lowermost
sample in
chron
Uppermost
sample in
chron
Stratigraphic
position of
base (m)
Stratigraphic
position of
top (m)
Chron
thickness
(m)
Uncertainty
(m)
A- C27r
Kutz Canyon KC45A KC13A 0.0 29.0 29.0 ± 0.5
Escavada Wash EM01A EM20C 0.0 38.0 38.0 ± 4.0
Torreon West TW01A TW14A 0.0 36.7 36.7 ± 0.6
Torreon East ET02A ET11D 0.0 31.8 31.8 ± 0.25
B+ C27n
Kutz Canyon KC15A KC39D 29.0 64.5 35.5 ± 2.75
Escavada Wash EM21A EM42C 38.0 72.3 34.3 ± 7.25
Torreon West TW15B TW23D 36.7 72.2 35.5 ± 0.8
Torreon East ET12A ET40C 31.8 57.5 26.0 ± 0.75
C- C26r
Kutz Canyon KC41A KC44D 64.5 69.0 4.5 ± 2.25
Escavada Wash EM45B EM50C 72.3 77.6 5.3 ± 3.25
Torreon West TW24A TW38C 72.2 78.4 6.2 ± 0.2
Torreon East ET18C ET35C 57.5 74.0 16.5 ± 0.5
Note: Uncertainty is the number of meters between samples of opposing polarity.
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0.2
FC, remanence
ZFC, remanence
30025020015010050
T[K]
RT remanence on cooling
RT remanence on warming
M(x10
-4
Am
2
/kg)
9.0
10.0
7.5
6.5
7.0
5.5
6.0
5.0
M(x10
-2
Am
2
/kg)
30025020015010050
T[K]
1.0
0.3
0.2
FC, remanence
ZFC, remanence
Ms (x10
-2
Am
2
/kg)
600500400300200100 700
T [°C]
0.2
0.4
1.3
2.0
RT remanence on cooling
RT remanence on warming
C15TW06A
black
30025020015010050
M (x10
-4
Am
2
/kg)
4.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
M(x10
-3
Am
2
/kg)
M(x10
-4
Am
2
/kg)
30025020015010050
T[K]
RT remanence on cooling
RT remanence on warming
F
C15ET11
brown
300 400
FC, remanence
ZFC, remanence
4.0
3.5
3.0
2.5
2.0
35025020015010050
3.5
3.0
2.5
2.0
1.5
1.0
M (x10
-4
Am
2
/kg)
T[K]
E
C15ET11
brown
4.0
4.5
T[K]
T[K]
Ms (x10
-2
Am
2
/kg)
600500400300200100
0
1
2
3
5
6
7
9
10
1.32
1.34
1.36
1.38
1.40
1.28
1.26
1.24
1.22
700
T [°C]
8
4
T[K]
300
1.42
1.30
M (x10
-2
Am
2
/kg)
25020015010050
C
C15TW23
red D
C15TW23
red
RT remanence on cooling
RT remanence on warming
9.5
8.5
8.0
4.5
1.3
1.2
1.1
0.9
0.8
0.7
0.6
0.5
0.4
0.1
T[K]
300250
M(x10
-3
Am
2
/kg)
RT remanence on cooling
RT remanence on warming
2.2
2.1
1.9
1.8
1.7
1.6
1.5
1.4
1.4
1.2
1.0
0.8
0.6
0
20015010050
C15TW06A
black
G
C15KC35
tan
H
C15KC35
tan
I
C15KC12
red
J
C15KC31
tan, silty
AB
3.0
3.2
3.4
3.6
3.8
30025020015010050
Fig. 6. Rock magnetic analysis results including low temperature magnetization curve of representative
samples and high temperature VSM (vibrating sample magnetometer) curves of saturation magnetization. Room
temperature (RT) plots show magnetization measurements upon cooling and warming between 20K and
room temperature (300K) following the application of saturation isothermal remanent magnetization (SIRM) at
room temperature. FC (field-cooled) and ZFC (zero field-cooled) plots show magnetization during warming
following a sustained direct current field of 2.5 T during cooling (FC), and magnetization during warming
following a SIRM imparted at low temperature (ZFC). (A) RT curves for a black, slickensided paleosol sample
indicating goethite and magnetite. (B) FC/ZFC curves for a black, slickensided paleosol sample indicating
goethite. (C) High temperature VSM curve for a red, calcareous paleosol sample indicating goethite, titanomag-
netite, and hematite. (D) RT curves for a red, calcareous paleosol sample indicating maghemite. (E) RT curves
for a brown paleosol sample indicating magnetite and goethite. (F) FC/ZFC curves from the goethite test for a
brown paleosol sample indicating goethite and titanohematite. (G) RT curves for a tan paleosol sample
indicating magnetite and goethite. (H) FC/ZFC curves for a tan paleosol sample indicating Al-substituted
goethite. (I) High temperature VSM curves for a red, calcareous paleosol indicating goethite, titanomagnetite,
magnetite, and hematite. (J) RT curves for a tan, silty weakly-developed paleosol sample indicating titanomagne-
tite and magnetite (see text for details).
316 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
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temperature. However, some magnetic remanence persists upon subsequent cooling
to 20 K indicating the presence of a high coercivity phase that is not demagnetized at
low temperature, nor up to 400 K, likely indicating the presence of titanohematite.
RTSIRM curves for a representative specimen of a tan paleosol suggests both
magnetite and goethite from the presence of the Verwey transition and an increase of
remanence upon cooling (fig. 6G). FC-ZFC-LTSIRM curves converge at 190 K, which
indicates Al-substituted or nano-goethite (fig. 6H). RTSIRM curves for a tan, silty
weakly-paleosol sample have a separation at 90 K indicating maghemite and a slight
Morin transition indicating hematite (fig. 6J).
Triaxial-IRM Lowrie tests were performed to determine the relative abundance of
magnetic carriers in each sample. For all samples analyzed the majority of the IRM is
held by grains whose coercivities are greater than 1 T (fig. 7). The largest remanence
drop occurs between 100 to 150 °C suggesting goethite is the most abundant mineral-
ogy in the samples. Some samples also show a decrease in remanence between 150 to
200 °C suggesting titanohematite may also be present. Remanence remaining after
200 °C is likely held by pigmentary hematite for samples TW23 and KC12, supported by
the loss of remanence between 600 to 650 °C, red coloring, and the low temperature
rock magnetometry experiments. In all other samples, the remaining remanence is
likely maghemite that inverted to hematite upon heating above ⬃450 °C. All samples,
Magnetization (x10-4Am2kg-1)
6
5
4
3
2
1
0
100 200 300 400 500 600
Temperature (°C)
Tota l
X=1T
Y=300mT
Z=100mT
TW06A - black
Magnetization (x10-4 Am2kg-1)
1.4
1.2
1.0
0.8
0.6
0.2
0
100 200 300 400 500 600
Temperature (°C)
Tota l
X=1T
Y = 300 mT
Z=100mT
ET11 - brown
1.6
Magnetization (x10-3Am2kg-1)
6
5
4
3
2
1
0
100 200 300 400 500 600
Temperature (°C)
Tota l
X=1T
Y = 300 mT
Z = 100 mT
TW23 - red
Magnetization (x10-3 Am2kg-1)
2.0
1.0
0.5
0
100 200 300 400 500 600
Temperature (°C)
Tota l
X=1T
Y = 300 mT
Z=100mT
KC12 - red
7
8
9
1.5
Magnetization (x10-3 Am2kg-1)
1.5
1
0.5
0
100 200 300 400 500 600
Tem per at ur e (°C )
Tota l
X=1T
Y=300mT
Z = 100 mT
KC31 - tan, silty
2
2.5
Magnetization (x10-3Am2kg-1)
0.8
0.6
0.4
0
100 200 300 400 500 600
Temperature (°C)
Tota l
X=1T
Y=300mT
Z=100mT
KC35 - tan
1.0
1.2
0.4
0.2
AB
CD
F
E
Fig. 7. Thermal demagnetization of orthogonal isothermal remanent magnetization (IRM) imparted
along X, Y, and Z axes for six specimens following Lowrie (1990) indicating a mixed magnetic mineralogy
with goethite as the dominant carrier.
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except TW06A, also show remanence held by the 300 mT curve up to ⬃300 °C,
indicating the presence of magnetite/maghemite.
Detrital Sanidine Dating
The age probability plot for sample HJ16-SJ-11 from Escavada Wash is shown in
figure 8. The sample is dominated by Paleocene grains where 66 of the 86 crystals give a
H16-SJ-11 detrital sanidine
omitted
Age (Ma)
Relative probability
%40Ar*
K/Ca
90
95
100
105
100
10
0
40
80
60 64 68 72 76
N
80
Age = 62.48 ± 0.02 Ma
MSWD = 9.8
n = 66 of 86
Fig. 8. Age probability diagram for the analyzed sample, H16-SJ-11. The plot shows, from bottom to top,
the age probability distribution spectrum for the data shown with solid squares, distribution of individual
single-crystal ages with 1␴errors, K:Ca ratios, and percentage of radiogenic 40Ar.
318 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
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weighted mean age of 62.48⫾0.02 Ma. Older grains concentrate around 68 and 70 Ma
with 8 crystals giving apparent ages ⬎200 Ma. The dominance of a high proportion of
grains near 62.5 Ma suggests that this sample may be a minimally reworked tephra layer
such that the maximum deposition age closely approximates the actual deposition age.
discussion
Magnetic Mineralogy
The upper Nacimiento Formation has a mixed magnetic mineralogy with titanohe-
matite and maghemite as the characteristic remanent magnetization carriers. Goethite
is also present in all lithologies and dominates the low-temperature magnetic measure-
ments, however it does not contribute to the characteristic remanence direction.
Titanohematite is likely present in most samples, supported by the drop in remanence
between 150 to 200 °C in the orthogonal IRM measurements and low temperature
magnetometry measurements. Figure 6F shows that during the goethite test, there is
an increase in remanence from room temperature to low temperatures after the
goethite is removed. The phase must be high coercivity because it persists after the
goethite test and the 1T orthogonal demagnetizations (fig. 7) and must have an
ordering temperature above 400 K since it survives heating to that point (fig. 6F).
These characteristics collectively indicate titanohematite as a characteristic remanent
magnetization carrier, in agreement with the observations of Butler and Lindsay
(1985). However, although titanohematite along with maghemite constitute the
primary magnetic mineralogy, titanohematite does not appear to be the most abun-
dant remanence carrier as suggested by Butler and Lindsay (1985). It is possible that
given the broad sample spacing in Butler and Lindsay (1985), our samples do not
overlap. Nonetheless, the presence of detrital titanohematite in the sections does
support their conclusion that sediments were derived from the volcanic San Juan
Mountains to the north. Goethite is the most abundant magnetic mineral and likely an
alteration product, whereas the titanohematite is likely detrital. Agreement of the
characteristic remanent directions obtained in this study to those reported by Kodama
(1997) for the Nacimiento Formation confirm the primary nature of the magnetiza-
tions residing in maghemite and titanohematite. Kodama (1997) performed an
inclination-correction study of the Formation, and reporteda7to8°inclination bias,
however, it should be noted that any inclination shallowing would not affect a reversal
stratigraphy.
Relationship of Polarity Stratigraphy to GPTS
Taylor and Butler (1980) reconstructed the magnetic polarity stratigraphy at
Torreon West, Torreon East, and Kutz Canyon and correlated the strata to C26r-C25r
based on previous work by Lindsay and others (1978) and Taylor (ms, 1977). Butler
and Lindsay (1985) revised the correlation of the polarity zones to the GPTS to
correlate the late To2-To3 deposits at Torreon Wash and Kutz Canyon with C27r
through C26r. We use the Butler and Lindsay (1985) interpretations for the base of
the Nacimiento Formation and correlate the A⫺,B⫹, and C⫺magnetozones from the
sections in our study to C27r-C26r (figs. 2 and 3). The polarity stratigraphy of the
To2-To3 strata of Escavada Wash has not been previously published. This section also
contains A⫺,B⫹, and C⫺magnetozones. A detrital sanidine age of 62.48⫾0.02 Ma
from 64 meters in the section, within the B⫹magnetozone, indicates correlation to
C27n (Ogg, 2012). Thus we can correlate the underlying A⫺interval to C27r and the
overlying C⫹interval to C26r.
Sediment Accumulation Rates and Section Durations
Given that the entire C27n polarity zone (local polarity B⫹) occurs in each section
and the temporal duration of the corresponding polarity chron has been estimated
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(Ogg, 2012), we were able to calculate mean sediment accumulation rates for each
section shown in table 4. Uncertainty associated with the duration of C27n (Ogg, 2012)
was taken from Cande and Kent (1992), where the uncertainty in the time scale was
assumed to be the same as the uncertainty in the width of the magnetic anomaly for
C27n, which was ⫾6.9 percent. Uncertainties for the calculated sediment accumula-
tion rates are asymmetrical because different stratal thicknesses and durations for
C27n were used. The maximum sediment accumulation rate was calculated by dividing
the maximum thickness of C27n strata by the minimum duration of C27n. The
minimum sediment accumulation rate was calculated by dividing the minimum
thickness of C27n strata by the maximum duration of C27n. Kutz Canyon, Torreon
West, and Escavada Wash have similar mean sediment accumulation rates ranging
from 115.7 to 119.9 m/myr and the rates are indistinguishable when factoring in their
uncertainties. Torreon East has a lower sediment accumulation rate of 87.0 m/myr
(⫺8.0, ⫹9.0 m/myr). Large channel bodies within the C27n interval of this section
suggest that erosion into the landscape occurred, reducing the overall preserved
sediment thickness and resulting in a lower calculated accumulation rate. As a result,
application of the calculated mean sediment accumulation rate to the remainder of
the section may overestimate the amount of time the section spans if less erosion or
more sediment accumulation occurred over the C27r and C26r intervals. The similar-
ity of sediment accumulation rates at Kutz Canyon, Escavada Wash, and Torreon West
suggests that sedimentation was similar across the basin as sampled sections are
located both near the basin center (Kutz Canyon) and southern edges (Torreon
West and Escavada Wash). This similarity is important because the Nacimiento
Formation thins from north to south in the basin. The work of Taylor (ms, 1977)
indicates that the entire C27r through C27n section is nearly twice as thick in Kutz
Canyon (150 m) as in Torreon West (58 m). Consequently, sediment accumulation
rates at Kutz Canyon for the lower part of the Nacimiento Formation (that is, early
in C27r) must have been much greater than in Torreon Wash, but equalize higher
in the section (Taylor, ms, 1977). This interpretation is supported by sediment
accumulation rates calculated for C27r in Kutz Canyon based on the magnetostratig-
raphy of Taylor (ms, 1977), which suggest mean rates of ⬃120 to 150 m/myr
compared to preliminary mean accumulation rates for C27r-C28n from Kutz
Canyon of ⬃180 m/myr (Peppe, unpublished data).
The duration of each section and the ages of all of the mammal fossil localities
within each section was determined by extrapolating each section’s C27n calculated
sediment accumulation rate to the top and the bottom of each section (fig. 9; tables 1
and 4). Uncertainties for the duration of each section are also asymmetrical because
the maximum and minimum sediment accumulation rates were used to calculate the
uncertainties. Kutz Canyon is estimated to span the least amount of time, 580 kyr (⫺90,
⫹100 kyr), from 62.76 to 62.18 Ma. Escavada Wash spans 750 kyr (⫺190, ⫹280 kyr)
from 62.85 to 62.10 Ma. Torreon West spans 650 kyr (⫺80, ⫹80 kyr) from 62.82 to
62.17 Ma. Torreon East is calculated to span 870 kyr (⫺100, ⫹100 kyr) from 62.88 to
62.01 Ma, but as noted above, the presence of large channel bodies within the C27n
interval suggests that erosion occurred and the calculated sediment accumulation rate
may be an underestimation for the remainder of the section, which results in an
overestimate of the duration of deposition in the Torreon East section.
Evolution of the San Juan Basin
The calculated sediment accumulation rates for C27n, which are considerably
lower than those for C27r, suggest that the basin was probably mostly filled prior to the
end of C27r (late To2) and sedimentation equalized across the basin during the end of
C27r through the start of C26r. These findings support the three phase subsidence
model proposed by Cather (2004) that determined a phase of subsidence from ⬃74 to
320 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
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TABLE 4
Calculated Sediment Accumulation Rates and Durations of Polarity Zones
Kutz Canyon Escavada Wash Torreon West Torreon East
C27n duration* (myr) 0.296 ± 0.02 0.296 ± 0.02 0.296 ± 0.02 0.296 ± 0.02
Calculated sediment
accumulation rate (m/myr)
119.9 (-16.3, +18.7) 115.7 (-30.3, +34.7) 119.9 (-10.1, +11.6) 87.0 (-8.0, +9.0)
Calculated duration of C27r strata
(myr)
0.24 (-0.03, +0.04) 0.33 (-0.10, +0.16) 0.30 (-0.03, +0.04) 0.36 (-0.04, +0.04)
Calculated duration of C26r strata
(myr)
0.04 (-0.02, +0.02) 0.12 (-0.05, +0.08) 0.05 (-0.01, +0.01) 0.21 (-0.02, +0.03)
Calculated age of section base
(Ma)
62.76 (-0.05, +0.06) 62.85 (-0.12, +0.18) 62.82 (-0.05, +0.06) 62.88 (-0.06, +0.06)
Calculated age of section top
(Ma)
62.18 (-0.04, +0.04) 62.10 (-0.10, +0.07) 62.17 (-0.03, +0.02) 62.01 (-0.05, +0.04)
Calculated total section duration
(myr)
0.58 (-0.09, +0.10) 0.75 (-0.19, +0.28) 0.65 (-0.08, +0.08) 0.87 (-.10, +.10)
* C27n duration from Ogg (2012), uncertainty was taken from Cande and Kent (1992), where the uncertainty in the time scale was assumed to be the same as uncertainty in the
width of the magnetic anomaly for C27n, which was ⫾6.9%. Uncertainties associated with the sediment accumulation rates, strata durations, section ages, and section durations are
shown in parentheses. Sediment accumulation uncertainties are asymmetrical because different stratal thicknesses and durations for C27n were used. The maximum sediment
accumulation rate was calculated by dividing the maximum thickness of C27n strata by the minimum duration of C27n. The minimum sediment accumulation rate was calculated by
dividing the minimum thickness of C27n strata by the maximum duration of C27n. The maximum and minimum sediment accumulation rates were used to calculate uncertainties for
durations and ages, also resulting in asymmetrical uncertainties.
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67 Ma resulted in the deposition of the Nacimiento Formation. Following this phase of
subsidence, the space available for sediments to fill likely decreased with time through-
out the early Paleocene so that during the late part of C27r through early in C26r
(⬃63–62 Ma) there was little accommodation space in the basin. This lack of
accommodation space likely caused the unconformity between the upper Nacimiento
62.2
62.3
62.5
62.662.462.1
62.7
62.8
Ma
C27r
C27n
C26r
A-
C-
B+
Kutz
Canyon
A-
C-
B+
Escavada
Wash
A-
C-
B+
Torreo n
West
A-
C-
B+
Torreo n
East
NALMA
interval
zone
5
6,7,8
9
12
13
14
15 16
17
18
10,11
To2
To3
U
U
M
MM
UML
U
M
L
1,2
3
4
49,50
51,52
43,44,
45
46,47,
48
53,54,
55,56
M
19
20
21
22
23
24
25
26
42
40,41
38,39
33,34,35
29,30,31
32
36,37
27,28
L
Fig. 9. Chronostratigraphy of the Upper Nacimiento Formation showing the age and calculated
duration of the Kutz Canyon, Escavada Wash, Torreon West, and Torreon East sections. Time scale from
Ogg (2012) is shown to the left. Lithologies are highlighted for each section along with fossil localities. Fossil
localities 1–4 from Kutz Canyon, 5–18 from Escavada Wash, 19 – 42 from Torreon West, and 43–56 from
Torreon East are described in table 1.
322 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
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Formation and the overlying Eocene San Jose Formation. Sedimentation resumed in
the Eocene once additional accommodation space was created, the third phase of
subsidence documented by Cather (2004).
This interpretation of equalized accommodation space in the San Juan Basin
plausibly explains the dominance of sheet sands rather than channels throughout the
top of C27r-C26r interval in all four sections (figs. 2 and 3) and the similar sediment
accumulation rates from basin center to basin margin (table 3). If accommodation was
low in the basin during this time, the sediment transport capacity of a channel was
likely frequently exceeded, resulting in unconfined flow and deposition of laterally
continuous sheet sands. This evidence for unconfined flow also suggests that the
Nacimiento Formation records the progradation and/or aggradation of a distributive
fluvial system (DFS), characterized by a radial channel pattern and aggradational
deposition below the point where flow becomes unconfined (Weissmann and others,
2010; Hobbs, ms, 2016). Low accommodation and fan progradation could also explain
the presence of better drained soils embodied by red, calcareous paleosols. Well-
developed soils are expected during accommodation minimums and fan progradation
because the distance to the water table increases and distance to the source decreases
(Weissmann and others, 2010; Atchley and others, 2013). However, it is important to
note that variations in climate may have also been an important contributing factor in
generating these features.
Mammalian Biostratigraphy
The Nacimiento Formation contains the only deposits in North America where
mammalian turnover between To2-To3 can be constrained, and temporal constraint of
this turnover may provide a better understanding of possible driving mechanisms.
Using estimated sediment accumulation rates (table 4), an age was assigned to each
To2 and To3 mammal locality in each section (table 1, fig. 9). Our magnetostratigra-
phy for the upper Nacimiento Formation corroborates the interpretation of Lofgren
and others (2004) that the To2 NALMA interval zone occurs within C27r and the To3
NALMA interval zone occurs within C27n. Additionally, there is no evidence that the
turnover is diachronous across the San Juan Basin.
Both late To2 and To3 mammals are found at Escavada Wash, Torreon West, and
Torreon East, and thus these areas are the best locations to determine the timing of the
To2-To3 turnover. At Escavada Wash the youngest To2 locality (L-10350) occurs at
62.71 Ma (⫺0.07, ⫹0.08 myr) and the oldest To3 localities (L-09981 and L-10444)
occur at 62.35 Ma (⫺0.10, ⫹0.05 myr), which indicates the turnover occurred over 360
kyr (⫺120, ⫹180 kyr). At Torreon West, the youngest To2 locality (L-08180) occurs at
62.59 Ma (⫺0.02, ⫹0.03 myr) and the oldest To3 locality (L-09173) occurs at 62.46 Ma
(⫺0.02, ⫹0.03 myr) for a duration of 130 kyr (⫺50, ⫹50 kyr) between NALMA interval
zones. At Torreon East the youngest To2 localities (L-07583, L-04954, and L-04950)
occur at 62.63 Ma (⫺0.02, ⫹0.04 myr) and the oldest To3 localities (L-10013 and
L-10432) occur at 62.47 Ma (⫺0.02, ⫹0.04 myr) for a duration of 160 kyr (⫺60, ⫹60
kyr) between the interval zones. When using the youngest To2 locality in the basin,
which is a locality in Torreon West (L-08180), and the oldest To3 localities, which are
localities in Torreon East (L-10013 and L-10432), the duration between the To2-To3
NALMA interval zones is 120 kyr (⫺60, ⫹50 kyr) and occurred between 62.59 to 62.47
Ma. This indicates that To2-To3 turnover occurred over a relatively short period of
time and suggests that the To2-To3 transition was probably not driven exclusively by
typical patterns of origination and extinction. Instead, they may be due to very rapid
rates of speciation, migration of new taxa into the basin, or both. The driver of an
increase in mammalian speciation rates and/or dispersal of taxa into the basin is
uncertain, but we posit that it was driven by external factors such as environmental
change associated with changes in basin dynamics or regional/global changes in
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climate. Further work focused on reconstructing the environment and climate of this
interval will help resolve this question.
conclusions
The polarity zones for the four sections in this study, Kutz Canyon, Escavada Wash,
Torreon West, and Torreon East, are correlated to chrons C27r-C26r of the geomag-
netic polarity time scale. This study is the first published polarity stratigraphy for the
Escavada Wash section, and a
40
Ar/
39
Ar detrital sanidine age further constrains the age
of the section. Rock magnetic analyses indicate that although goethite is the most
abundant magnetic mineral, titanohematite and maghemite carry the characteristic
remanent magnetization. The age model for the upper Nacimiento Formation pre-
sented here has important implications for the evolution of the San Juan Basin and the
timing and nature of the To2-To3 mammalian turnover. Our results indicate that
sediment accumulation rates near the basin center were similar to those near the basin
edge, which signals that from the end of C27r through the start of C26r sediment
accumulation rates had equalized across the basin. This suggests that the deposition of
the upper Ojo Encino Member occurred during a basin accommodation minimum,
which was associated with the progradation of a distributive fluvial system across the
basin. The later unconformity between the Paleocene Nacimiento Formation and
the lower Eocene San Jose Formation was likely driven by a lack of accommodation in
the San Juan Basin and sedimentation could not resume until additional accommoda-
tion space was created in the Eocene. This is consistent with Cather’s (2004) three
phase model of basin subsidence.
Estimates of sediment accumulation rates for strata in the Escavada, Torreon West
and East, and Kutz Canyon sections were used to determine the age to each To2-To3
fossil locality in those sections, and in turn constrain the duration of the To2-To3
turnover. These results indicate that the To2-To3 turnover was rapid and occurred
over ⬃120 kyr (⫺60, ⫹50 kyr) between 62.59 and 62.47 Ma. The rapid nature of the
mammalian turnover suggests that there was likely an external forcing factor, such as
an environmental change driven by the progradation of the distributive fluvial system
across the basin and/or changes in regional or global climate.
ACKNOWLEDGMENTS
This work was supported by SEPM (CL), National Science Foundation EAR-
132552 (DP), ⫺1325612 (RS), ⫺0207750 and ⫺1325544 (TEW), American Chemical
Society Petroleum Research Fund (PRF#52822-DN18) (DP), Baylor University Depart-
ment of Geosciences’ Dixon Fund (CL, TL), and the University of Minnesota Institute
for Rock Magnetism visiting student fellowship (CL). This is IRM publication #1706.
We thank K. Kodama, D. Lofgren, and one anonymous reviewer for their helpful
comments and P. Gingerich for the editorial assistance. Thanks to A. Flynn, A. Davis, J.
Milligan, and A. Baumgartner for assistance with sample collection and section
measuring.
324 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
rich4/zqn-ajsc/zqn-ajsc/zqn00318/zqn2410d18z
xppws S⫽1 3/30/18 3:23 Art: zqn-2410

APPENDIX 1
Paleomagnetic data: lines
Location
Paleosol
#
Strat
level (m)
Sample
Temp. steps
Strat.
Declination
Strat.
Inclination
MAD
Strat. VGP
latitude
Strat. VGP
longitude
Kutz
P34
1
C15KC45A
200,250,275
174.3
-39.3
19.1
-74.9
272
Kutz
P34
1
C15KC45B
250-300
197.2
-68.1
18.1
-70.9
106.5
Kutz
P34
1
C15KC45C
250-300
163.8
-26
4
-63
288.7
Kutz
P35
2
C15KC46C
250-300
136.3
-29.2
11.7
-46
325.5
Kutz
P35
2
C15KC46D
200-250
146
-33.8
8.3
-55.2
320.3
Kutz
P36
2
C15KC47A
225,250,300
127.6
-73.2
16.6
-49.7
32.8
Kutz
P37
4.5
C15KC48A
250-300
178
-64.2
9.9
-80.4
63.7
Kutz
P37
4.5
C15KC48B
250-300
157.4
-69.8
8.3
-66.8
36.8
Kutz
P37
4.5
C15KC48C
150,225,250
125.3
-65
12.6
-48.7
14.5
Kutz
P1
15.5
C15KC01B
225-275
171.9
-41.5
9.3
-75.6
283.2
Kutz
P1
15.5
C15KC01D
250-300
189.4
-24.1
18.8
-64.7
230.2
Kutz
P1
16
C15KC02A
175, 250, 275
251.3
-56.2
20
-34.2
138.7
Kutz
P1B
16
C15KC02B
150-225
173.1
-58.2
5.4
-84
7.8
Kutz
P1
16
C15KC02D
250-275
129.6
-62.3
5.8
-51.4
8.7
Kutz
P2
18.5
C15KC03A
150-200
217.7
-29.9
9.8
-50.9
183.5
Kutz
P3
19
C15KC04A
175, 200, 250
65.9
-62.5
8.3
-10.1
30.1
Kutz
P3
19.5
C15KC05A
150-200
167.8
-28.6
7.6
-66.1
282.4
Kutz
P4
20.5
C15KC06A
150-200
138.9
-24.8
3.1
-46.4
320.3
Kutz
P5
21.8
C15KC07B
150-200
145.9
-50.5
12.4
-61.4
341.6
Kutz
P5
21.8
C15KC07C
250-300
171.7
-32.9
4.9
-70.0
275.8
Kutz
P5
21.8
C15KC07D
125-175
160.3
-23.6
16.7
-060.0
293.3
Kutz
P6
24.8
C15KC08C
175-225
221.2
-59.4
9.3
-57.8
142.7
Kutz
P7
26.3
C15KC09A
250-300
116.7
-31.1
19.3
-31.2
340.5
Kutz
P7
26.3
C15KC09C
275-325
197
-28.5
14.9
-63.9
212.3
Kutz
P7
26.5
C15KC10A
250-300
183.7
-25.9
11.1
-66.9
242.9
Kutz
P7
26.5
C15KC10B
250-300
191.1
-18.1
11
-61
229
Kutz
P7
26.5
C15KC10D
250-300
190.9
-35
19.1
-70.3
220.1
Kutz
P8
27.8
C15KC11B
225-300
171.6
-13.9
12.3
-59.6
268.7
Kutz
P8
27.8
C15KC11D
150-200
220.7
-46.4
7.7
-54.7
164.6
Kutz
P9
27.5
C15KC12A
250-300
176.7
-46.2
4
-80.6
270.3
Kutz
P9
27.5
C15KC12C
250-300
182.7
-40.4
10.1
-76.4
241.5
Kutz
P9
27.5
C15KC12D
250-300
186.5
-46.9
9.2
-79.3
245.8
Kutz
P10
28.5
C15KC13A
250-300
141.9
-55.2
14.7
-59.4
352.2
Kutz
P11
29.5
C15KC15A
250-300
103.4
74.8
13.4
25.7
283.1
Kutz
P11
29.5
C15KC15B
NRM, 125, 150
331.7
38.5
4.1
61.3
138.8
Kutz
P11
29.5
C15KC15D
NRM, 125, 150
355.3
44.4
7.5
78.8
94.5
Kutz
P11
30.5
C15KC16A
NRM, 125, 150
22.4
73.4
7.3
63.1
277.6
Kutz
P11
30.5
C15KC16B
NRM, 125, 150
350.6
62.3
3.7
79.9
209.7
Kutz
P11
30.5
C15KC16C
125, 225, 275
340.2
47.4
5.3
71.6
142.2
Kutz
P12s
31.5
C15KC17A
150-200
334.7
44.1
15.2
66
143
Kutz
P12
31.5
C15KC17B
250-300
351.6
17
15.8
61.2
89.5
Kutz
P12
31.5
C15KC17D
125-175
355.5
53.2
2.8
85.4
126.7
Kutz
P13
32.5
C15KC18A
250-300
16.9
65.7
3.7
73.1
294.3
Kutz
P13
32.5
C15KC
150-200
2.4
63.2
5.1
81.6
263.8
Kutz
P13
32.5
C15KC18D
225-275
349
58.2
4.5
81
180.7
Kutz
P13
32.8
C15KC19A
250-300
357.1
47.6
2.9
81.8
90.3
Kutz
P13
32.8
C15KC19B
225-275
352.4
45.6
13.8
78.6
108.5
Kutz
P13
32.8
C15KC19C
200-250
24.6
58.3
6.2
70.4
327.3
Kutz
P14
33.5
C15KC20B
175,225,275
1.5
54.7
5.2
88.2
27.9
Kutz
P14
33.5
C15KC20C
125,175,200
347.8
56.7
4.1
80.2
170.3
Kutz
P14
33.5
C15KC20D
125,150,200
5.7
45.6
8.2
79.4
43.4
Kutz
P14
33.8
C15KC21A
200, 275, 325
274.7
74.5
9
33.5
216.7
Kutz
P14
33.8
C15KC21B
150-200
337.6
18.1
20
56
114.4
Kutz
P14
33.8
C15KC21C
NRM, 125, 200
23.6
47
9.7
68.5
358
Kutz
P15
35.5
C15KC22A
125,175,200
2.2
50.7
4.7
84.6
51.7
Kutz
P15
35.5
C15KC22D
NRM, 125, 150
264.5
82.3
15.6
33.6
233.9
Kutz
P16
36.2
C15KC23A
225,250,300
354
31.3
14.3
69.7
88.9
Kutz
P16
36.2
C15KC23D
125-175
14.6
30.3
8.9
66
35.6
Kutz
P16
36.5
C15KC24A
225,250,300
14
54.5
4.4
78.6
345.3
Kutz
P16
36.5
C15KC24C
225-275
354.6
44.5
6.7
78.7
97.6
Kutz
P16
36.5
C15KC24D
225,275,300
11.3
49.9
4.8
49
10.5
Kutz
P17
39.7
C15KC25C
125, 175, 225
4.2
34.4
13.8
72
59.1
Kutz
P18
39.3
C15KC26C
NRM, 125, 175
10.1
36.9
11.4
71.8
40.4
Kutz
P19
39.8
C15KC27D
125-175
341.7
69.4
5.3
69.1
220.2
Kutz
P20
41.5
C15KC28A
125-175
349.3
43.8
11.7
75.8
115.1
Kutz
P20
41.5
C15KC28C
250-300
332.9
40.1
16.5
62.9
139.3
Kutz
P21
43
C15KC29B
125, 200, 225
25.2
27.7
7.3
58.7
19.6
Kutz
P21
43
C15KC29C
125, 175, 325
332.2
59.6
15.2
68
180.9
Kutz
P22
44
C15KC30C
NRM, 125, 250
73.2
69.2
8.8
37.9
299.3
Kutz
P22
44
C15KC30D
NRM, 125, 150
10.8
21.9
13
63
48.2
Kutz
P23
44.6
C15KC31C
125-175
359.4
36.1
13.4
73.5
74.1
Kutz
P24B
46
C15KC32B
125, 200, 225
16
77.7
14.3
58.7
264.3
Kutz
P24
46
C15KC32C
250-300
346
63
12
76.7
203.3
Kutz
P24
46
C15KC32D
125, 175, 200
359.6
65.1
4.8
79.4
250.6
325San Juan Basin, New Mexico, USA: Implications for basin evolution and mammalian
rich4/zqn-ajsc/zqn-ajsc/zqn00318/zqn2410d18z
xppws S⫽1 3/30/18 3:23 Art: zqn-2410

APPENDIX 1
(continued)
Location
Paleosol
#
Strat
level (m)
Sample
Temp. steps
Strat.
Declination
Strat.
Inclination
MAD
Strat. VGP
latitude
Strat. VGP
longitude
Kutz
P25
47
C15KC33D
100-150
70.3
64.5
9.6
38.1
307.8
Kutz
P26
47.8
C15KC34B
250-300
346.8
12.8
12.8
76.5
131.1
Kutz
P26
47.8
C15KC34C
250-300
12.6
38.4
10
71.5
32.7
Kutz
P26
47.8
C15KC34D
250-300
348.7
51
9.2
79.5
138.2
Kutz
P27
48.5
C15KC35A
100, 275, 300
316.5
61.8
4.3
56.3
186.9
Kutz
P27
48.5
C15KC35D
200, 250, 275
354.4
53
9.9
84.6
131.1
Kutz
P28
56.5
C15KC36B
100-200
2.6
63
6.1
81.8
265.2
Kutz
P28
56.5
C15KC36D
NRM-150
322.6
44.2
13.1
56.6
154.4
Kutz
P29
57.8
C15KC37A
100, 200, 225
18.4
30.2
6.6
64
28.4
Kutz
P30
58.4
C15KC38A
225-275
156.1
-23
14.2
-57.4
299.4
Kutz
P30
58.4
C15KC38C
NRM, 100, 350
322.7
53.1
10.6
59.5
169.4
Kutz
P31
62.3
C15KC39A
NRM, 100, 225
328.5
67.5
5.7
63.5
203.7
Kutz
P31
62.3
C15KC39B
NRM, 100, 250
17.9
51.4
9
74.6
353.6
Kutz
P31
62.3
C15KC39D
NRM-150
45.1
57.9
9.2
54.5
324.8
Kutz
P32
66.8
C15KC41A
200, 300, 325
185.5
-26.3
15.4
-66.9
238.4
Kutz
P32
66.8
C15KC41C
125, 150, 250
84.5
-66.3
9.7
-23.3
26.4
Kutz
P32
67
C15KC42A
125-200
149.9
-68.8
5.8
-63.7
28.2
Kutz
P32
67
C15KC42C
150-225
128.9
-70.1
12
-51.1
25.5
Kutz
P32
67
C15KC42D
150-225
37.3
-69
10
-4.7
50.4
Kutz
P33
68.5
C15KC43A
200-275
122.4
-49.7
9.9
-42.3
352.5
Kutz
P33
69
C15KC44A
250-300
208.5
-62.2
15.9
-67.2
135.4
Kutz
P33
69
C15KC44C
250-300
168.2
-66.2
16.9
-75.2
40.2
Kutz
P33
69
C15KC44D
225-275
148.6
-49
8
-63
337.1
Torreon West
P22
29.4
C15TW01A
225-275
178.3
-63.2
1.3
-81.2
64.7
Torreon West
P22
29.4
C15TW01B
225-275
198.6
-61.8
3.9
-74.1
130.9
Torreon West
P22
29.4
C15TW01C
225-275
179.6
-64.4
2.7
-79.8
71
Torreon West
P24
31.5
C15TW02A
225-275
167.3
-65.8
1.7
-74.7
38.7
Torreon West
P24
31.5
C15TW02C
225-275
163.3
-55.6
3.8
-76.5
348.1
Torreon West
P24
31.5
C15TW02D
225-275
169.7
-44.9
3.3
-77.1
298.2
Torreon West
P24
31.7
C15TW03B
300, 350-375
167.4
-18.8
13.7
-61.3
279.2
Torreon West
P24
31.7
C15TW03C
225-275
195
-54
3.6
-77.7
165
Torreon West
P24
31.7
C15TW03D
225-275
172.2
-53.1
2.1
-83.2
324.8
Torreon West
P24
31.9
C15TW04A
225-275
180.3
-50.9
11.6
-85.6
249.3
Torreon West
P24
32.1
C15TW05C
225, 250, 325
171.2
-26.5
8.7
-66.6
274.6
Torreon West
P24
32.3
C15TW06A
225-275
125.6
-77.4
11.2
-46.8
43.6
Torreon West
P24
32.8
C15TW07A
125, 150, 225
167
-32.9
6.3
-68.6
288.5
Torreon West
P24
32.8
C15TW07C
225-275
147.8
-40
11
-59.2
326.1
Torreon West
P24
33.3
C15TW08A
NRM, 150, 175
170.5
-31.8
7.3
-69.4
279.2
Torreon West
P24
33.3
C15TW08B
715, 225, 275
132.9
-36.9
5.2
-46.2
335.1
Torreon West
P24
35.2
C15TW11C
200-250
219.6
-37.1
4.7
-52.3
175.6
Torreon West
P24
35.2
C15TW11D
225-275
221.4
-45.6
13.4
-53.9
165
Torreon West
P25
35.5
C15TW12B
125-275
203.9
-37.2
9.8
-64.2
192.2
Torreon West
P25
35.8
C15TW13D
100-150
38.2
72.8
6.8
56.6
288.8
Torreon West
P26
36.1
C15TW14A
125, 200, 300
174.8
-36.5
19.8
-73.7
270.2
Torreon West
P26
37.3
C15TW15B
225-275
1
20.8
7
64.7
70.3
Torreon West
P26
38.2
C15TW16D
125, 250, 225
296.2
51.7
13.2
38
178.3
Torreon West
P26
39
C15TW17B
150, 200, 250
353.6
45.1
17.5
79.2
104.6
Torreon West
P27
40
C15TW18C
150, 200, 225
12.3
31.5
9.4
68.1
39.5
Torreon West
P29
43.2
C15TW19A
100, 125, 175
8
39.2
4.8
74.5
43.7
Torreon West
P29
43.2
C15TW19B
100-150
342.2
59.7
4
75.3
186
Torreon West
P29
43.2
C15TW19C
225-275
309.9
48.7
9.9
47.9
168.8
Torreon West
P39
56.7
C15TW20A
225-275
355.2
63.6
1.5
80.1
232.5
Torreon West
P39
56.7
C15TW20C
225-275
358.7
60
3.4
85
241.2
Torreon West
P39
56.7
C15TW20D
225-275
359.6
66.5
3
77
251.4
Torreon West
P40
61.4
C15TW21A
125, 175, 200
16
52.7
3.9
76.6
349.6
Torreon West
P40
61.4
C15TW21B
225-275
353.2
58
8.8
84
190.9
Torreon West
P40
61.4
C15TW21C
175-225
296
72.5
5.2
43.3
211.4
Torreon West
P43
66.7
C15TW22A
225-275
0.2
80.5
5
54.5
252.7
Torreon West
P43
66.7
C15TW22B
125, 150, 225
350
55.7
1
81.9
167.3
Torreon West
P43
66.7
C15TW22D
175-225
341.2
50
4.1
73.5
149.6
Torreon West
P49
72
C15TW23A
125, 250, 275
5.2
43.3
2.4
78.3
48.7
Torreon West
P49
72
C15TW23C
100-150
355.2
42.8
4.1
78.1
94.2
Torreon West
P49
72
C15TW23D
100-150
349.6
67.3
4.5
74
227.7
Torreon West
P49
72.4
C15TW24A
100-150
5.2
-74.6
4.8
-7.2
70.1
Torreon West
P50
72.7
C15TW25C
225-275
131
-49.5
13.4
-49
349.3
Torreon West
P50
73.3
C15TW27D
125, 275, 200
152.8
-75.2
15.3
-58.9
48.2
Torreon West
P50
73.8
C15TW28A
175, 200, 250
230.1
-51.2
10.1
-48.7
153.3
Torreon West
P50
73.8
C15TW28B
175, 275, 325
302.4
-79.7
19.9
-23.9
91
Torreon West
P50
74
C15TW29B
100-150
160.3
-42.9
10.7
-69.8
314.8
Torreon West
P51
74.6
C15TW30A
250, 275, 325
196.7
-36.8
14.2
-68.7
204.8
Torreon West
P51
74.6
C15TW30B
175-225
7.5
73.5
8.4
66.1
262.1
Torreon West
P52
75.9
C15TW32A
300, 325, 375
173.2
-25.7
18.4
-66.7
269.5
Torreon West
P52
76.8
C15TW36C
150-200
149.1
-48.9
15.6
-63.5
338.1
Torreon West
P53
77.4
C15TW37B
225-275
170.4
-28.6
7.2
-67.5
277.5
326 C. Leslie and others—High-resolution magnetostratigraphy of the Upper Nacimiento Formation,
rich4/zqn-ajsc/zqn-ajsc/zqn00318/zqn2410d18z