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Giant Induan oolite: A case study from the Lower Triassic Daye Formation in the western Hubei Province, South China

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Most Phanerozoic oolites are marked by ooids with a diameter less than 2 mm. Observations on a Neoproterozoic oolite have resulted in a change of concept. The term “pisolite” that traditionally referred to oolites with a grain size of more than 2 mm, is now restricted to those coated carbonate grains formed by meteoritic freshwater diagenesis; oolites with a grain size of more than 2 mm are now defined as “giant”. Particular unusual giant oolites within a set of oolitic-bank limestones with thicknesses of more than 40 m in the top part of the Lower Triassic (Induan) Daye (Ruiping) Formation at the Lichuan section in the western part of Hubei Province in South China, represent an important sedimentological phenomenon in both the specific geological period and the geological setting that is related to the end-Permian biological mass extinction. Like the giant oolites of the Neoproterozoic that represent deposits where oolites formed in a vast low-angle carbonate ramp at that special geological period, the Triassic Daye Formation at the study section are significant because they provide a comparative example to help understand the evolving carbonate world reflected by oolites, the origin of which is still uncertain, and they give insight into the sedimentation pattern of the desolate sea floor, which resulted from the mass extinction at the turn of the Permian into the Triassic.
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RESEARCH PAPER
Giant Induan oolite: A case study from the Lower
Triassic Daye Formation in the western Hubei
Province, South China
Mingxiang Mei
a,b,
*
, Jinhan Gao
b
a
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
b
School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
Received 1 July 2010; accepted 9 November 2011
Available online 25 February 2012
KEYWORDS
Carbonate sedimentology;
Oolite diversity;
Triassic;
Induan;
Hubei Province;
South China
Abstract Most Phanerozoic oolites are marked by ooids with a diameter less than 2 mm. Observations
on a Neoproterozoic oolite have resulted in a change of concept. The term “pisolite” that traditionally
referred to oolites with a grain size of more than 2 mm, is now restricted to those coated carbonate grains
formed by meteoritic freshwater diagenesis; oolites with a grain size of more than 2 mm are now defined
as “giant”. Partic ular unusual giant oolites within a set of oolitic-ba nk limestones with thicknesses of
more than 40 m in the top part of the Lower Triassic (Induan) Daye (Ruiping) Formation at the Lichuan
section in the western part of Hubei Province in South China, represent an important sedimentological
phenomenon in both the specific geological period and the geological setting that is related to the
end-Permian biological mass extinction. Like the giant oolites of the Neoproterozoic that re present
deposits where oolites formed in a vast low-angle carbonate ramp at that special geological period, the
Triassic Daye Formation at the study section are significant because they provide a comparative example
to help understand the evolving carbonate world reflected by oolites, the origin of which is still uncertain,
* Corresponding author. State Key Laboratory of Geological Processes
and Mineral Resources, China University of Geosciences, Beijing 100083,
China. Tel.: þ86 13701326033.
E-mail address: meimingxiang@263.net (M. Mei).
1674-9871 ª 2012, China University of Geosciences (Beijing) and Peking
University. Production and hosting by Elsevier B.V. All rights reserved.
Peer-review under responsibility of China University of Geosciences
(Beijing).
doi:10.1016/j.gsf.2011.11.017
Production and hosting by Elsevier
available at www.sciencedirect.com
China University of Geosciences (Beijing)
GEOSCIENCE FRONTIERS
journal homepage: www.elsevier.com/locate/gsf
GEOSCIENCE FRONTIERS 3(6) (2012) 843e851
and they give insight into the sedimentation pattern of the desolate sea floor, which resulted from the mass
extinction at the turn of the Permian into the Triassic.
ª 2012, China University of Geosciences (Beijing) and Peking University. Production and hosting by
Elsevier B.V. All rights reserved.
1. Introduction
Oolites are marked by both an even lamination of the oolitic
cortex and the general absence of biogenic features; they are
distinct from oncolites, which are another type of coated carbonate
grains (Tucker and Wright, 1990; Mei et al., 1997; Siewers, 2003).
The smooth rings of the oolitic cortex are probably the most
reliable criterion to distinguish oolites from oncolites, and
biogenic features such as mucus films are evident in many oolites
(Newell et al., 1960 ; Fl
ugel, 2004). There is a clear but arbitrary
size distinction between oolites and pisolites, with the term oolite
referring to grains smaller than 2 mm and pisolites being those
oolitic grains larger than 2 mm. However, the term “pisolite” is
also used by many sedimentologists to refer to nonmarine oolite-
shaped grains that are formed by meteoric fresh-water diagenesis
(e.g. Fl
ugel, 2004). Observation on and detailed study of the
‘giant’ oolites in the Neoproterozoic have led to a change in the
usage of these terms (Summer and Grotzinger, 1993; Grotzinger
and James, 2000), so that now the term giant oolite refers to
large (>2 mm) oolitic grains formed in a marine environment
(Siewers, 2003; Fl
ugel, 2004).
In general, recent oolites tend to be less than 1 mm in grain
size (Tucker and Wright, 1990; Mei et al., 1997; Siewers, 2003).
Although there are some exceptions, most oolites throughout the
Phanerozoic have grain sizes smaller than 2 mm. Archean and
Proterozoic oolites tend to be slightly larger but are still
predominantly less than 2 mm in grain size (Grotzinger and
James, 2000). So far, giant oolites that have been described and
studied are preferably developed in the Neoproterozoic (Summer
and Grotzinger, 1993; Grotzinger and James, 2000). For a set of
oolitic-bank limestones that is developed in the top part of the
Triassic Daye Formation (or the Ruiping Formation of the Dye
Group; Chen and Jin, 1997; Yang et al., 2000) at the Lichuan
section in the western part of Hubei Province, South China
(see Fig. 1), most of ooids have a grain size of more than 2 mm
and represent a particular giant oolite occurrence in the Phaner-
ozoic. These oolitic limestones formed during a special geological
period, i.e., the Induan, and in a special sedimentary setting, i.e.,
on a ramp carbonate platform after a major transgression and
severe biological mass extinction at the turn of the Permian to
Triassic in South China (Chen, 1995; Hallam and Wignall, 1999;
Mei and Tucker, 2007; Mei et al., 2007). Thus, similar to the giant
oolites of the Neoproterozoic (Summer and Grotzinger, 1993;
Grotzinger and James, 2000), several features, which include the
special forming geological time period, the special sedimentary
setting, and the unusual appearance of giant oolites throughout the
Phanerozoic in general, endow the Triassic giant oolites in the
study area with an important sedimentological significance.
2. Geological background
In South China, oolitic limestones are well developed in the
Triassic (Induan) Daye Formation (Fig. 1); the Daye Formation is
equivalent to the Ruiping Formation of the Daye Group (Wu et al.,
1994; Chen and Jin, 1997; Yang et al., 2000). According to the
studies of Wu et al. (1994), the carbonate strata with the devel-
opment of oolitic limestones that are defined as the Daye
Formation are distinct from the littoral strata made up of sand-
stones and mudstones of the Induan Feixianguan Formation in the
western part of the Upper Yangtze region close to continental
areas. Wu et al. (1994) have also demonstrated that the oolitic-
grained bank underwent a progradation process toward the east
throughout the Induan age. With this special background, a set of
oolitic-bank limestones with a thickness of about 50 m were
developed in the top part of the Daye Formation at the Lichuan
section in the western part of Hubei Province (Fig. 2).
According to the international stratigraphic standard (Chen and
Jin, 1997; Yang et al., 2000; Gradstein et al., 2004; Zhang et al.,
2009), the duration of the Induan is from 251.0 0.4 Ma to
249.5 Ma. At the Lichuan section, the Ruiping Formation is
Figure 1 Simplified map showing Induan (Early Triassic) paleo-
geography and sedimentary facies of the Upper Yangtze region and
the location of the studied section (arrowed). Lithofacies codes are:
1 Z nearshore sandstone; 2 Z offshore sandstones and mudstone;
3 Z shallow-ramp limestone; 4 Z oolitic-bank limestone;
5 Z middle- to deep-ramp limestone; 6 Z shelf to basin marl and
shale. (1) to (4) Z developing periods of oolitic bank throughout the
Induan (proposed by Wu et al., 1994). Siliciclastic facies,
1e2 Z Feixianguan Formation, carbonate facies; 3e5 Z Daye
Formation, shelf facies; 6 Z Luolou and Qingyan formations. The
south is part of the Dianqiangui or Nanpanjiang Basin (Mei and
Tucker, 2007; Mei et al., 2007).
M. Mei, J. Gao / Geoscience Frontiers 3(6) (2012) 843e851844
marked by a set of limestones with some oolitic grainstones in its
upper part. Long-term research on the paleontology has provided
good chronostratigraphical control for the Daye Formation
(Chen and Jin, 1997; Yang et al., 2000) in the Induan, which
includes biostratigraphic zonations as follows: 1) bivalve zones
from the lower to the upper, i.e. Towapteria scythicum, Pseudo-
claraia wangi, Claraia aurita and Eumorphotis multiformis zones;
2) ammonite zones from the lower to the upper, i.e. Ophiceras-
Lytophiceras, Gyromites-Prionolobus, Koninckites-Xenodiscoides
and Plemingites zones; and 3) conodont zones from the lower to
the upper, i.e. Hinduodus minutus, Isarciella isarcica, Neo-
gondollella carinata, Neospathodus dieneri, Neospathodus pecu-
liaris and Neospathodus pakistanensis zones. Furthermore, this set
of oolitic-bank limestones was formed in the latest period of the
Induan age, which belongs to the product of the fourth period of
oolitic-bank development in the Upper Yangtze region proposed
by Wu et al. (1994), as shown (4) in Fig. 1.
Considering the sequence-stratigraphic division for the coeval
strata in adjacent areas, together with the underlying Permian Dalong
Formation and the overlying basal part of the Jialingjiang Formation,
the Daye Formation at the Lichuan section can be subdivided into two
third-order sequences, i.e., DS
1
and DS
2
(Fig. 2). The basic features
of these sequences can be summarized as follows:
(1) The basal boundary of DS
1
equates to a clear drowning-
unconformity, exhibited by basin siliceous rocks of the Permian
Dalong Formation that lie directly over high-stand carbonates of
the Upper Permian Changxing Formation (Schlager, 1989, 1999;
Chen, 1995; Hallam and Wignall, 1999; Mei and Tucker, 2007;
Mei et al., 2007); this larger-scale rapid transgression that
occurred at the turn of the Permian to the Triassic, formed an
upward-shallowing succession of sedimentary facies from basin
to shallo w ramp making up DS
1
;
(2) An abrupt transfer boundary of sedimentary facies from
shallow to deep ramp in the middle part of the Daye
Formation forms the basal boundary of DS
2
, which is marked
by a transitional type of sequence-stratigraphic boundary as
defined by Vail et al. (1977), and an exposure-punctuated
surface formed in the lower part of the Jialingjiang Forma-
tion, which is similar to the type-II sequence-stratigraphic
boundary defined by Vail et al. (1977), between which an
upward-shallowing succession of sedimentary facies from
deep ramp to top oolitic-bank constitutes DS
2
. Consequently,
a set of oolitic-bank limestones with a thickness of about
50 m makes up the early high-stand system tract (EHST), and
a set of dolomites with some dolomitic limestones and few
oolites belonging to the lower Jialingjiang Formation makes
up a forced-regressive wedge system tract (or late high-stand
system tract (LHST); Hunt and Tucker, 1992) of the third-
order sequence DS
2
.
3. Macroscopic features of the Triassic giant oolite
As seen in Fig. 3, for the thick-bedded to massive oolitic grain-
stones in the top part of the Daye Formation at the Lichuan
section, the macroscopic features can be summarized as follows:
(1) Large-scale wedge-shaped cross bedding is developed within
the oolitic grainstones;
(2) Ooids are arranged by denseness and those with a grain size
of more than 2 mm are frequently concentrated on the
Figure 2 Log showing the sequence-stratigraphic division for the
Lower Triassic Daye Formation at the Lichuan section, Hubei Province,
South China. DS
1
and DS
2
refer to two third-order sequences discerned
from the Permian Dalong Formation thru to the lower part of the
Triassic Jialingjiang Formation at the Lichuan section. SB Z sequence
boundary of transitional type; SB
2
Z type II; SB
3
Z drowning-
unconformity type of sequence boundary. Compositional units for
third-order sequences are: CS Z condensed section, EHST Z early
high-stand system tract, LHST Z late high-stand system tract. Lith-
ofacies codes are: 1 Z siliceous rock; 2 Z black shale; 3 Z medium- to
thin-bedded micrite; 4 Z medium- to thick-bedded aphanitic micrite;
5 Z oolitic grainstone; 6 Z dolomite and dolomitic limestone. Stars
represent the stratigraphic position of the oolitic limestone. The section
location is the same as that as shown in Fig. 1.
M. Mei, J. Gao / Geoscience Frontiers 3(6) (2012) 843e 851 845
scouring surface (Fig. 3A); there are three-dimensional ooids
visible in some parts of the outcrop in which some giant
oolites are distributed (Fig. 3B);
(3) Some particular oolitic intraclastics resulted from the breakup
of oolites caused by strong currents;
(4) Larger ooids with grain sizes of more than 2 mm (some from
4 to 5 mm) represent an unusual feature of giant oolites
throughout the Phanerozoic, which are similar to those in
the Neoproterozoic (Swett and Knoll, 1989; Summer and
Grotzinger, 1993; Grotzinger and James, 2000; Mei, 2008).
Therefore, several features, such as the development of large-
scale wedge-shaped cross bedding, the large thickness of a single
bed that can be up to several meters (2e4 m), and the development
of giant oolite and oolitic intraclasts, reflect the environment of
formation for Triassic oolites at the Lichuan section in a high-
energy agitating subtidal environment, i.e., the grain bank facies in
the top part of the EHST unit of DS
2
, as seen in Fig. 2.
4. Microscopic features of the Triassic giant oolite
Under the light microscope, there is a diversity of ooid
morphologies for the oolite from the upper Daye Formation at the
Lichuan section. The oolites are mainly composed of carbonate
micrite (Fig. 4; see detailed description by Mei, 2008). Their
microscopic features can be summarized as follows:
(1) Most of the ooids are circular (Fig. 4A and B), elliptical
concentric (Fig. 4E), irregular eccentric (Fig. 4D), and some
are composite (Fig. 4C). The ooid grain sizes are commonly
more than 2 mm, with a few 4e5 mm; and the largest can be
up to 7 mm (Fig. 4B and E);
(2) Many types of small carbonate grains, such as pellets (Fig. 4A
and B), calcisiltite (Fig. 4C), echinoderm bioclast (Fig. 4E),
silt-sized thrombolite with worm microscopic fabric (Fig. 4D)
and particular ooid intraclast (Fig. 4C), make up the nucleus of
the ooids. Furthermore, as seen in Fig. 4E, the echinoderm
bioclasts that form the ooid nucleus have an obvious micriti-
zation feature in their exteriors margin that may be genetically
related to microboring caused by euendolithic cyanobacteria
(Chac
on et al., 2006; Garcia-Pichel, 2006; Duguid et al., 2010),
and these echinoderm bioclasts are coated by a micritic enve-
lope in the form of a ring that is possibly formed by microbes;
(3) The ooid cortex is made up of even rings of dark micrite,
which exhibit dark and light laminations;
(4) The thickness of the ooid cortex is always larger than the size of
the nucleus, and the number of ooid rings is more than several
tens, which reflects the basic feature of high-energy oolites;
(5) As seen in Fig. 4C, the nucleus of one large irregular-shaped
composite ooid is made up of two small ooids, and this in turn
is finally wrapped by even ooid rings, which result in a composite
that is distinct from other aggregate grains such as grapestones
and thrombolites; furthermore, this large composite reflects the
basic feature of a rebirth ooid (Simone, 1981; Fl
ugel, 1982,
2004; Tucker and Wright, 1990; Mei et al., 1997; Siewers, 2003);
(6) As seen in Fig. 4D, an eccentric ooid with a diameter of more
than 3 mm indicates the basic feature of a rebirth ooid is similar
to the feature reflected by a large irregular-shaped composite
ooid seen in Fig. 4C, which may be the product of syndeposi-
tional deformation and a discontinuity during the growing
process of the ooids (Simone, 1981; Fl
ugel, 1982, 2004; Tucker
and Wright, 1990; Mei et al., 1997
; Si
ewers, 2003).
As seen in Fig. 5, the infillings of th e ooids include cements of
calcite spar and intraclasts. The vari ous ty pes of ooids shown in
Fig. 4 probably belong to high-energy oolites (Simone, 1981;
Fl
ugel, 1982, 2004; Tucker and Wright, 1990; Mei et al., 1997;
Siewers, 2003). The particula r filling substances are those ooid
intraclasts with cle ar ooid-ring microfabrics (Fig. 5A), which
resulted from fragmentation and abrasion of oolites caused by
strong currents during formation. For these calcite-spar cements
(Fig. 5B), the first generation is marked by fibrous calcites, the
second generation characterized by fibrous and flaky calcite, and
the third generation is made up of e quably grained blocky
calcites with an obvious micropore about 0.1 mm across.
Micropores within blocky calcite cements are partly concentrated
in the cent ral parts of po res among the ooids and are open
through e ach other, which reflect obvious corrosion during
diagenesis.
As mentioned above, the precise original mechanism of oolite
formation is poorly understood. However, a number of processes
have been put forward (Gerdes et al., 1994; Reid et al., 2000;
Brehm et al., 2003, 2006; Duguid et al., 2010), which include:
1) biological formation, biological processes and organisms cause
precipitation of calcium carbonate; 2) chemical formation,
Figure 3 Photographs showing the macroscopic features for the
Triassic oolites in the study area. Photo A demonstrates a giant oolite
with dense distribution in the scouring surface and the development of
large-scale cross bedding within the oolitic grainstone; Photo B
exhibits ooids in three-dimensional view.
M. Mei, J. Gao / Geoscience Frontiers 3(6) (2012) 843e851846
calcium carbonate is induced to precipitate around the nuclei
when it supersaturates in an aqueous solution; 3) physical
formation, through the accretion of calcium carbonate while the
grain is being rolled around on the sea floor. It is likely, therefore,
that a combination of these processes can be responsible for ooid
formation, although microbial processes are emphasized by
Gerdes et al. (1994) and by Brehm et al. (2003, 2006), and
chemical processes by Duguid et al. (2010).
As seen in Fig. 4, the giant oolites of the Induan in the study
area demonstrate a complex formation process that is related to
Figure 4 Images showing various giant oolite morphologies within the oolitic grainstones in the top part of the Triassic Daye Formation at the
Lichuan section, Hubei Province. Images (A) to (B) are circular concentric ooids, (C) is a composite ooid, (D) is an irregular eccentric ooid, and
(E) is an elliptical concentric ooid. All images were taken under plane-polarized light.
M. Mei, J. Gao / Geoscience Frontiers 3(6) (2012) 843e 851 847
the precipitation of amorphous calcium carbonate (ACC) caused
by microbial activity. Firstly, as seen in Fig. 4AeD, the cortex of
the ooids with various morphologies demonstrates dark and light
laminations are similar to the basic feature of stromatolites, which
may indicate that these oolites are the product resulting from
a spherical microbial assemblage as proposed by Brehm et al.
(2003, 2006). Secondly, some ooid rings are composed of dark
amorphous micrites enriched with organic substances, as seen in
Fig. 4, and this type of ooid ring has a thickness of several tens of
microns that are similar to a micritic envelope, which demon-
strates that the formation mechanism for this type of ooid ring is
similar to the dark laminations of stromatolites and is genetically
related to microbial activity. Thirdly, the basic feature of rebirth
oolites as seen in Fig. 4C and D shows that the growth of the ooid
rings might be controlled by a complex precipitation process of
carbonate micrites that are genetically related to the activity of
microbes, and this feature cannot simply be interpreted with the
chemical formation process of carbonate micrites proposed by
Duguid et al. (2010). Fourthly, as seen in Fig. 4E, the clear
micritization feature in the exterior margin of an echinoderm
bioclast that constitutes an ooid nucleus might be genetically
related to a microboring caused by euendolithic cyanobacteria
(Chac
on et al., 2006; Garcia-Pichel, 2006; Duguid et al., 2010).
Therefore, the formation mechanism for these oolites (e.g. Figs. 3
and 4) is directly or indirectly related to specialized microbial
activity, as proposed by Gerdes et al. (1994) and Brehm et al.
(2003, 2006).
5. Significance of Triassic giant oolites
Several features of the ooids within the Triassic oolitic limestones
at the Lichuan section, i.e., the formation in the early high-stand
period of third-order sea level change (DS
2
in Fig. 2; Tong et al.,
1999), and during the progradational process of an oolitic bank
toward the east depositing on a carbonate ramp (Fig. 1; Wu et al.,
1994), the morphological diversity of ooids (Figs. 3 and 4), the
special ooid intraclasts indicating abrasion and fragmentation
during formation (Fig. 5A), and the multigenerational calcite spar
cement (Fig. 5B), indicate that these oolites were generated in
a high-energy subtidal environment (e.g., Reeder and Rankey,
2008). The Lower Triassic Daye Formation strata of the
carbonate ramp at the Lichuan section formed in a shallower
sedimentary environment than the outer platform paleoenviron-
ment where the Induan Luolou Formation of the Nanpanjiang
Basin was deposited (e.g., Galfetti et al., 2008). Although the
environmental conditions for the formation of the oolitic grain
remain uncertain (Simone, 1981; Fl
ugel, 1982, 2004; Tucker and
Wright, 1990; Mei et al., 1997; Siewers, 2003), the top Daye
Formation oolitic grainstones reflect a favorable sedimentary
background for the generation of oolites in the last part of the
Induan.
The special conditions that are the key to the formation of
these giant oolites showing similarities to those in the Neo-
proterozoic included:
(1) A high-energy environment developed on the shallow ramp
under strong action of tidal and wave currents during the
formation process of the ramp carbonate platform favorable to
the formation of high-energy oolites ( Simone, 1981; Fl
ugel,
1982, 2004; Tucker and Wright, 1990; Mei et al., 1997;
Siewers, 2003; Reeder and Rankey, 2008);
(2) Strong agitation of the depositional water bodies in the high-
energy environment, which can result in frequent reworking
of sedimentary grains on the sea floor, and thus be favorable to
the generation of oolites (Simone, 1981; Fl
ugel, 1982, 2004;
Tucker and Wright, 1990; Mei et al., 1997; Siewers, 2003;
Reeder and Rankey, 2008). The resultant oolites (e.g. Figs. 3
and 4) are representative of ooids of pelagic (‘suspended’)
origin (Gerdes et al., 1994; Brehm et al., 2003, 2006);
(3) Remoteness from siliclastic sources (e.g. Fig. 1), which can
generate clear water with few siliceous elements that is most
conduci
ve to the generation of oolites;
(4) The underlying strata of the top Daye Formation oolitic
limestones at the Lichuan section are marked by aphanitic and
laminated micritic limestones, which reflect low generation
and provision rates of bioclasts, which in turn would have led
to a concomitant low rate of ooid nucleuses.
As noted above, the observation of Neoproterozoic giant
oolites (Summer and Grotzinger, 1993; Grotzinger and James,
2000) led to a change in concept so that the term “pisolite” that
was traditionally used to refer to large oolites with a diameter of
more than 2 mm has now been restricted to refer to the ooid-like
non-marine carbonate grains formed by freshwater diagenesis.
Ooids with a diameter of more than 2 mm have been defined
Figure 5 Images showing oolitic intraclasts (A) and multigenera-
tional cements of calcite spar (B). Aeshows angular oolitic intraclasts
with a clear microscopic fabric to the ooid cortex; Be demonstrates
the calcite spar cement within the inter-grain pores in which there are
some clear micropores in the central part (arrowed). All images were
taken under plane-polarized light.
M. Mei, J. Gao / Geoscience Frontiers 3(6) (2012) 843e851848
consequently as those of a giant oolite (Siewers, 2003; Fl
ugel,
2004). The giant Neoproterozoic oolites have the largest size
with more than 14 mm, and most are 8e9 mm in diameter, which
is instructive to consider variation in ooid size. For the Neo-
proterozoic examples, Summer and Grotzinger (1993) gave
a reasonable interpretation for their origin, which included
a combination of the following: 1) a higher growth rate due to
higher carbonate saturation of seawater; 2) lower nucleation rate
imparted by a low flux of nuclei; 3) increased storminess due to
the prevalence of ramps and the possibly stormier weather in an
erratic climate; 4) increased absolute level of storminess that was
genetically related to the waxing and waning of the extensive
Neoproterozoic ice sheet.
Considering that the giant Neoproterozoic oolites occur strati-
graphically below tillite deposits formed during the glaciation
(Swett and Knoll, 1989; Knoll and Swett, 1990) or between glacial
periods (Singh, 1987), and that the giant oolites are absent in most
similar settings of Phanerozoic age such as the Neogene icehouse,
the increased agitation cannot be the sole catalyst for the develop-
ment of giant oolites (Grotzinger and James, 2000). Although there
are some uncertainties for the origin of the Neoproterozoic giant
oolites, they do provide an important analogy for the further
understanding of the Triassic giant oolites in the study area.
The formation age of the Triassic oolites at the Lichuan section
during the last period of the Induan is just the time of recovery
of biota after the end-Permian biological mass extinction
(Schubert and Bottjier, 1992; Wignall and Twitchett, 1999; Pruss
et al., 2004; Hips and Hass, 2006; Liu et al., 2007), which means
that there are peculiarities in the sedimentary and environmental
settings of this special time in the study area: 1) the development
of vast epicontinental areas that were characterized by a low-angle
carbonate ramp resulting from the breakup of Pangea (Santosh,
2010); 2) that ramp is dominated by neritic lime-mud deposi-
tion, along with peloids, ooids and minor amounts of bioclasts,
equivalent to the skeleton-poor sea of the Early Cambrian (Pruss
et al., 2010); 3) a special geological period that is dominated by
sedimentation of microbial carbonates and respective deposits
(Schubert and Bottjier, 1992; Wignall and Twitchett, 1999; Pruss
et al., 2004; Hips and Hass, 2006; Kershaw et al., 2007; Liu et al.,
2007; Mei, 2007), such as stromatolites (Schubert and Bottjier,
1992), unusual edgewise intraclasts (Wignall and Twitchett,
1999), and wrinkle structure (Pruss et al., 2004), indicating
among other things the development of microbial mats on the
depositional surface.
As seen in Fig. 2, the Daye Formation represents the devel-
opment of a low-angle ramp platform with an area of m ore than
one million square kilometers (Wu et al., 1994; Enos et al.,
2005), in which a sedimentary setting that is favorable to the
generation of oolites is developed. Given this setting, increased
storminess due to the prevalence of ramps and possibly
a stormier climate mi ght h ave led to the development of the
agitating water conditions
needed for the generation of oolites;
further, the development of a neritic lime-mud “ factory” (Pomer
and Hallock, 2008) in the recovery period after biological
extinction could have led to low generation/provision rates of
oolitic nucleuses conducive to the formation of the giant oolite.
Thus, the formation environment of the Triassic oo lites has
many similarities to that of the giant Neoproterozoic oolites
(Swett and Knoll, 1989; Knoll and Swett, 1990; Summer and
Grotzinger, 1993; Grotzinger and James, 2000), including
neritic lime-mud production, a marine environ ment with rela-
tively high seawater alkalinity, low production rate of bioclasts,
and the agitating environment. Ultimately, together with the
special stromatolite and other microbial carbonates (Schubert
and Bottjier, 1 992; Hips and Hass, 2006; Kershaw et al., 2007;
Mei, 2007), the unusual edgewise intraclasts (Wignall and
Twitchett, 1999), the wrinkle structure indicating the develop-
ment of microbial mats (Pruss et al., 2004) and so on, b oth the
diversity of oolitic morphology and t he development of an
unusual giant oolite in the Phanerozoic in the study area reflect
a devasted sea-floor environment (Schubert and Bottjier, 1992;
Wignall and Twitchett, 1999 ; Fang, 2004; Pruss et al., 2004;
Hips and Hass, 2006; Kershaw et al., 2007; Liu et a l., 2007; Wu
et al., 2007) or else a new kind of anachronistic facies (Pruss
et al., 2004; Li et al., 2010) in the aftermath of the end-
Permian mass extinction.
Although the origin of oolites remains uncertain, their
morphological diversity with the development of an unusual giant
oolite in the Phanerozoic provides an important example to help
us understand more about the evolution of the carbonate world
represented by giant oolites (Wu, 1992; Grotzinger and James,
2000; Fl
ugel, 2004; Yan and Wu, 2006; Pomer and Hallock,
2008). Furthermore, this important example also provides an
important clue to further research. Importantly, some of the
microbial features for the Triassic giant oolite, as seen in Fig. 4,
such as the relatively thick ooid rings composed of dark amor-
phous micrites enriched with organic substances and nucleuses
made up of echinoderm bioclasts with reworking of microbial
borings, may also represent a type of post-mass-extinction disaster
form in addition to microbialites, edgewise intraclasts, wrinkle
structures and so on. If so, they act as a proxy for a high-energy
agitating skeleton-poor sea that is favorable to the formation of
giant oolites directly or indirectly controlled by microbial
activities.
6. Conclusions
Both the diversity of ooid morphology and the development of the
oolitic grainstones in the top part of Lower Triassic Daye
Formation at the Lichuan section, Hubei Province, not only
provide an example of the development of unusual giant oolites in
the Phanerozoic but also provide an important clue to study further
evolving sedimentary response to biological mass extinction in the
carbonate world. These oolitic limestones formed during a special
geological period, i.e., the Induan, after the most severe biological
mass extinction of the Phanerozoic, and in a special sedimentary
setting, i.e., on a ramp carbonate platform after a major trans-
gression, somehow similar to Neoproterozoic giant oolites.
Although the natural formation conditions for a single ooid
remains uncertain, the observations on the Triassic giant oolites in
the study area also provide important clues toward understanding
this conundrum of sedimentology, i.e., the original mechanism of
oolites.
Acknowledgments
This research was funded by the Natural Sciences Foundation of
China (Grant Nos. 49802012, 40472065) and is a contribution to
IUGS: UNESCO IGCP 572. We benefited from field discussions
with Dr. Liuqin Chen, Dr. Kaibo Duan and Dr. Huo Rong.
Acknowledgments are also due to Prof. Jinnan Tong, the coordi-
nator of IGCP 572, for his support in this research. Dr. Susan
Turner (Brisbane) assisted with English.
M. Mei, J. Gao / Geoscience Frontiers 3(6) (2012) 843e 851 849
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... Ooids, mainly calcareous, spherical to subspherical coated grains consisting of one to several nearly concentric cortices encrusting a nucleus, are ubiquitous features of (sub) tropical oceans throughout Earth's history (Flügel, 2004;Tucker, 2011). Although there is no commonly accepted elucidation for ooid genesis, processes such as accretion of fine particles around a nucleus while agitating on a soft substrate (Mei and Gao, 2012), abiotic precipitation from an ambient supersaturated water around a nucleus (Duguid et al., 2010) and organomineralization of a surface biofilm (Diaz et al., 2014;Li et al., 2017;Batchelor et al., 2018) have been diversely put forward. Ancient ooids are 2 | VARKOUHI And JAQUES RIBEIRO valuable palaeoenvironmental proxies for water energy, temperature, salinity, seawater geochemistry and bathymetry (after Sandberg, 1975;Plee et al., 2008). ...
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... Lower Jurassic carbonate platforms of western Tethys (Mei & Gao, 2012;Ettinger et al. 2021). Accordingly, the Pliensbachian-Toarcian extinction is associated with a sequence boundary related to transgression (Hallam, 1997;Haq, 2018), which drowned the carbonate platform and therefore affected the carbonate factory. ...
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... 4e and 4f) and echinoderm/bivalve debris along with thick cortices-several tens of lamellas-which reflect the basic feature of high-energy ooids (Figs. 4c to 4g; Mei & Gao, 2012). 3. Compound ooids: This type of ooids is rare and appears as two or three (superficial) ooids enveloped by a thin cortex of several lamellas (Figs. ...
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... Ooids have been reported from diverse marine carbonate strata ranging across the Archean to the Phanerozoic (Tucker, 1985;Wilkinson et al., 1985;Swett and Knoll, 1989;Wright and Altermann, 2000;Armella et al., 2007;Mei and Gao, 2012;Ramkumar et al., 2013;Antoshkina, 2015;Tang et al., 2015). However, existence of ooids through the earth's geological history has emphasized maximum concentration of giant ooids in Proterozoic carbonates of marine origin. ...
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This study is the first comprehensive documentation of giant ooids and related facies associations in the Kunihar Formation, Proterozoic Simla Group Lesser Himalaya, Himachal Pradesh, analysing the understanding of processes and controls on development of giant ooids. Based on field observations, supplemented by outcrop based facies analysis, petrography and delineation of environmental variations, four facies associations have been delineated: (i) Peritidal siliciclastic-carbonate (FA1) (ii) Shelf lagoon (FA2) (iii) Reef complex (FA3) (iv) Fore reef slope (FA4). Deposition of giant ooids and associated facies associations of Kunihar Formation occurred in a carbonate rimmed shelf with high tidal influence. Size of giant ooids from Kunihar Formation is the largest as compared to giant ooids from other geological formations. Kunihar giant ooids developed when normal ooids were washed from ooid shoals (intertidal) into slightly deeper regions (shallow subtidal) resulting in the increased dimension of ooids in suspension due to higher hydrodynamics. Scanning electron microscopy (SEM) studies support microbial origin of ooids, giant ooids and stromatolites of Kunihar Formation. Increased microbial activity in Kunihar Formation is attributed to increase in nutrients by virtue of weathering of underlying Darla volcanics. Abundant carbonate and Microbially induced sedimentary structures (MISS) deposition in lower part of Simla Group points to increased microbial activity which likely increased the volume of oxygen in Neoproterozoic atmosphere ushering in Ice House conditions during the subsequent deposition of Blaini Group. Giant ooids associated with Neoproterozoic glacial deposits throughout the world, occupy stratigraphic positions below, above or between glaciations. Simla Group is another example where giant ooids lie stratigraphically below Marinoan Blaini Tillites. Increased magmatic activity and weathering before and during the Neoproterozoic glaciations increased nutrients in marine waters which increased algal growth. Thus, giant ooids were deposited due to such phases of increased microbial activity before/after glaciation, and during interglacial periods.
... However, field investigations and thin section observations show fewer fossils but abundant ooids and oncoids that are clearly different from previously documented, highly fossiliferous Permian reef-associated limestones. In South China, oolitic and oncolitic limestones were widely developed on the Yangtze Platform during the Early Triassic following the end-Permian mass extinction (Rong et al., 2010;Mei and Gao, 2012;Li et al., 2013). Therefore, the limestone block from Bayan Har should be Lower Triassic in age according to regional stratigraphic correlations and petrographic analysis. ...
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The Bayan Har area, situated in the northeast of the Qinghai-Tibetan Plateau, has long been regarded as a deep-water turbidite basin during Early Triassic. However, a sequence of Lower Triassic shallow-water carbonate rocks has been documented from the region, indicating the existence of isolated seamount carbonate platforms in the basin. The seamount carbonate platform sediments differ from synchronous deep basin flysch sediments, and consist of shallow-water oncoids, ooids and cortoids. Microfacies studies reveal that the oncoids have a thick cortex comprised of irregular, non-concentric and partially overlapping micritic laminae and contain abundant well-preserved foraminifera, indicating a relatively low-energy environment in subtidal settings. Ooids exhibit thinner and more regular concentric laminations, which are interpreted to have formed by frequent overgrowth while rolling in a relatively high-energy setting. Cortoids are predominantly composed of tiny fossil fragments and intraclasts with a micritic envelope, indicating a shallow-marine warm water environment located in the intertidal or supertidal zone. Ooids in the Bayan Har area differ from those found on stable platforms, in that they sometimes nucleated onto volcanic quartz grains, reflecting an unstable tectonic setting that was frequently affected by volcanic activity. Widespread ooids and other microbial carbonates formed in the Early Triassic have been regarded anachronistic facies that are indicative of harsh marine conditions. Most Lower Triassic anachronistic facies are distributed along the shallow margin of the Tethys Ocean and the western margin of Pangea; however, Bayan Har was located in the center of the Palaeo-Tethys. The discovery of oncoids and oolites capping a seamount in this area provides strong evidence that extraordinary marine conditions spread far into the interior of the Palaeo-Tethys Ocean.
... Ooids (\2 mm in diameter) and ''giant ooids'' ([2 mm in diameter; may be also termed pisoids, but pisoids commonly refer to a freshwater or terrestrial origin) are spherical, concentric, coated grains (Li et al. 2010;Mei and Gao 2012) (Fig. 10). There are a variety of ooids in the Fig. 10 Microbial ooids. ...
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This study illustrates features of the Cambrian oncoids and provides a comparison with other microbial-related carbonate grains found in the Cambrian succession of the North China epeiric platform. Based on cortex structures, four types of oncoids were distinguished: thin-cortex (superficial) oncoids, laminated-cortex oncoids, clotted-cortex oncoids, and full-cortex (without nucleus) oncoids. Thin- and clotted-cortex oncoids are often associated with oolites, laminated-cortex oncoids are present within oolitic-bioclastic grainstones, and full-cortex oncoids occur in bioturbated wackestones. The oncoids with nucleus–cortex structures are easily distinguished from other carbonate grains due to the lack of nucleus–cortex structures, and from microbial-related ooids which have more circular shape and more continuous cortex than oncoids. Oncoids without nucleus and with only crudely laminated cortex (i.e., full-cortex oncoids) can be differentiated from microbialite intraclasts and microbial lumps by the following evidences: (1) microbialite intraclasts, either rounded or angular, are characterized by margins that sharply truncate the included calcified microbes or carbonate grains and, in addition, intraclast-bearing conglomerates commonly show clear sedimentary structures such as cross-stratification and normal grading; (2) microbial aggregates have irregular but smooth margins, and rather chaotic inner structures.
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The end‐Permian extinction and its aftermath altered carbonate factories globally for millions of years, but its impact on platform geometries remains poorly understood. Here, the evolution in architecture and composition of two exceptionally exposed platforms in the Nanpanjiang Basin are constrained and compared with geochemical proxies to evaluate controls on platform geometries. Geochemical proxies indicate elevated siliciclastic and nutrient fluxes in the basal Triassic, at the Induan‐Olenekian boundary, and in the uppermost Olenekian. Cerium/Ce* shifts from high Ce/Ce* values and a lack of Ce anomaly indicating anoxia during the Lower Triassic to a negative Ce anomaly indicating oxygenation in the latest Olenekian and Anisian. Uranium and Mo depletion represents widespread anoxia in the world’s oceans in the Early Triassic with progressive oxygenation in the Anisian. Carbonate factories shifted from skeletal in the Late Permian to abiotic and microbial in the Early Triassic before returning to skeletal systems in the Middle Triassic, Anisian coincident with declining anoxia. Margin facies shifted to oolitic grainstone in the Early Triassic with development of giant ooids and extensive marine cements. Anisian margins, in contrast, are boundstone with a diverse skeletal component. The shift in platform architecture from ramp to steep, high‐relief, flat‐topped profiles is decoupled from carbonate compositions having occurred in the Olenekian prior to the onset of basin oxygenation and biotic stabilisation of the margins. A basin‐wide synchronous shift from ramp to high‐relief platforms points to a combination of high subsidence rate and basin starvation coupled with high rates of abiotic and microbial carbonate accumulation and marine cement stabilisation of oolitic margins as the primary causes for margin up‐building. High seawater carbonate saturation resulting from a lack of skeletal sinks for precipitation, and basin anoxia promoting an expanded depth of carbonate supersaturation, probably contributed to marine‐cement stabilisation of margins that stimulated the shift from ramp to high‐relief platform architecture.
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This paper reviews the current progress and problems in the study of microbialites and microbial carbonates. Microbialites and microbial carbonates, formed during growth of microbes by their calcification and binding of detrital sediment, have recently become one of the most popular geological topics. They occur throughout the entire geological history, and bear important theoretical and economic significances due to their complex structures and formative processes. Microbialites are in place benthic microbial buildups, whereas microbial carbonates can be classified into two categories: stabilized microbial carbonates (i.e., carbonate microbialites, such as stromatolites and thrombolites) and mobilized microbial carbonates (i.e., microbial carbonate grains, such as oncoids and microbial lumps). Various texture, structures, and morphologies of microbialites and microbial carbonates hamper the systematic description and classification. Moreover, complex calcification pathways and diagenetic modifications further obscure the origin of some microbialites and microbial carbonates. Recent findings of abundant sponge spicules in previously identified “microbialites” challenge the traditional views about the origins of these “microbialites” and their implications to reef evolution. Microbialites and microbial carbonates did not always flourish in the aftermath of extinction events, which, together with other evidences, suggests that they are affected not only by metazoans but also by other geological factors. Their growth, development, and demise are also closely related to sea-level changes, due to their dependence on water depth, clarity, nutrient, and sunlight. Detailed studies on microbialites and microbial carbonates throughout geological history would certainly help understand causes and effects of major geological events as well as the coevolution of life and environment.