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Sedimentology and Paleontology of the Upper Cretaceous Wahweap Formation sag ponds adjacent to syndepositional normal faults, Grand Staircase-Escalante National Monument, Utah
Abstract
During the development of the East Kaibab monocline, listric normal faulting related to outer arc influenced the sedimentation style of the Upper Cretaceous Wahweap Formation. The initiation of the Laramide Orogeny was therefore recorded in the sedimentary record of south-central Utah. Evidence for this includes the preservation of sag ponds adjacent to two of the normal faults in our study area, which developed when fault movement created topographic features. Ancient sag-pond deposits are likely under-identified in the rock record. This study demonstrates their significance and potential for unraveling fault histories.
The northern, younger, sag-pond deposit is located at the boundary between the upper and capping sandstone members of the Wahweap Formation, and consists of sandstone dikes and sills hosted in gray mudstones and siltstones with no discernable invertebrate fauna, but some small macerated flora. The second southern sag-pond fill, is located at the base of the upper member. The sag pond is older, larger in area, and contains a thicker deposit. In contrast to the northern sag-pond deposit siltstones and mudstones are gray and visiblity structureless. Within the southern sag pond there are a series of fossil horizons consisting mainly of juvenile unionid bivalves, a lesser number of gastropods, and macerated plants. Comparison of the two preserved Upper Cretaceous sag-pond deposits suggests two distinct responses to fault movement, perhaps governed by fault kinematics, manifested in sedimentation style and type; the impact of faunal invertebrate invasion; and post-sedimentation deformation.
... Laramidestyle, basement-involved uplifts exist on the margins of the SWHP, including the Kanarra Fold to the west, and the East Kaibab monocline and Circle Cliffs uplifts to the east (Fig. 1). The earliest evidence of this foreland break-up in southern Utah includes growth faults in the Campanian Wahweap Formation in the northern Kaiparowits Plateau (e.g., Hilbert-Wolf et al., 2009;Simpson et al., 2014). One of the main drivers for Laramide tectonism is flat-slab subduction, which is thought to have initiated at the latitude of southern Utah during the earliest Campanian (Simpson et al., 2014). ...
... The earliest evidence of this foreland break-up in southern Utah includes growth faults in the Campanian Wahweap Formation in the northern Kaiparowits Plateau (e.g., Hilbert-Wolf et al., 2009;Simpson et al., 2014). One of the main drivers for Laramide tectonism is flat-slab subduction, which is thought to have initiated at the latitude of southern Utah during the earliest Campanian (Simpson et al., 2014). Inverse mantle convection models show the flat-slab segment, likely the conjugate of the Shatsky Rise, entering the subduction zone along the Mojave area of California at ca. 90 Ma, followed by northeast underthrusting beneath Arizona to Wyoming during the latest Cretaceous Liu and Currie, 2016). ...
Anomalous features of Upper Cretaceous strata in southern Utah challenge existing tectonic and depositional models of the Cordilleran foreland basin. Extreme thickness variations, net to gross changes, and facies distributions of nonmarine to marginal marine strata of the Turonian−early Campanian Straight Cliffs Formation are documented across the Southwestern High Plateaus. Contrary to most traditional models of foreland basin architecture, regional correlations demonstrate abrupt stepwise thickening, with a punctuated increase in average grain size of key intervals from west to east, i.e., proximal to distal relative to the fold-thrust belt. Except in the most proximal sections, fluvial drainage systems were oriented predominantly subparallel to the fold-thrust belt. Combined, these results suggest that modern plateau-bounding faults may have had topographic expressions as early as Cenomanian time, and influenced the position of the main axial river system by creating northeast-trending paleotopography and sub-basins. Laramide-style tectonism (e.g., basement-involved faults) is already cited as a driver for sub-basin development in latest Cretaceous−Cenozoic time, but new data presented here suggest that this part of the foredeep was “broken” into distinct sub-basins from its earliest stages. We suggest that flexural foundering of the lithosphere may have caused early stage normal faulting in the foredeep. Regional implications of these new data indicate that both detachment-style and basement-involved structures were simultaneously active in southern Utah earlier than previously recognized. These structures were likely influenced by inherited Proterozoic basement heterogeneities along the edge of the Colorado Plateau. This interpretation suggests that tectonic models for the region should be reevaluated and has broader implications for understanding variability and geodynamics of foreland basin evolution.
... Laramidestyle, basement-involved uplifts exist on the margins of the SWHP, including the Kanarra Fold to the west, and the East Kaibab monocline and Circle Cliffs uplifts to the east (Fig. 1). The earliest evidence of this foreland break-up in southern Utah includes growth faults in the Campanian Wahweap Formation in the northern Kaiparowits Plateau (e.g., Hilbert-Wolf et al., 2009;Simpson et al., 2014). One of the main drivers for Laramide tectonism is flat-slab subduction, which is thought to have initiated at the latitude of southern Utah during the earliest Campanian (Simpson et al., 2014). ...
... The earliest evidence of this foreland break-up in southern Utah includes growth faults in the Campanian Wahweap Formation in the northern Kaiparowits Plateau (e.g., Hilbert-Wolf et al., 2009;Simpson et al., 2014). One of the main drivers for Laramide tectonism is flat-slab subduction, which is thought to have initiated at the latitude of southern Utah during the earliest Campanian (Simpson et al., 2014). Inverse mantle convection models show the flat-slab segment, likely the conjugate of the Shatsky Rise, entering the subduction zone along the Mojave area of California at ca. 90 Ma, followed by northeast underthrusting beneath Arizona to Wyoming during the latest Cretaceous Liu and Currie, 2016). ...
... A series of Late Cretaceous syndepositional, listric normal faults, linked to the onset of the Laramide orogeny, are exposed in the east-dipping limb of the East Kaibab monocline in Grand Staircase-Escalante National Monument, southern Utah ( Fig. 1; Tindall et al., 2010). These faults were seismically active during deposition of the upper and capping sandstone members, and evidence includes growth strata (significantly thicker sequences on the downthrown side of the fault), colluvial breccia of recycled Wahweap sediment, distribution and intensity of soft-sediment deformation (seismites) and sag pond deposits (Fig. 2;Hilbert-Wolf et al., 2009;Simpson et al., 2009;Tindall et al., 2010;Simpson et al., 2013Simpson et al., , 2014 At Bull Flat, the metre-scale load structures are found over an area greater than a thousand square metres, adjacent to the fault and along the sandstone-sandstone contact of the upper and capping sandstone members. The contact dips about 25°and is well-exposed along a dipparallel slope (Fig. 2). ...
... Bull Flat is located along the tip line of the fault and later fault activity may have propagated the fault through the load. Numerous lines of evidence indicate multiple phases of fault movement, including multiple stratigraphic intervals of seismites that can be attributed to the normal faults , growth faults and synorogenic strata (Tindall et al., 2010), and sediment fill within sag pond deposits (Simpson et al., , 2014. ...
Large-scale soft-sediment deformation structures occur within fluvial sandstone bodies of the Upper Cretaceous Wahweap Formation in the Kaiparowits basin, southern Utah, USA. These structures represent an exceptional example of metre-scale fault-proximal, seismogenic load structures in nearly homogenous sandstones. The load structures consist of two types: large-scale load casts and wedge-shaped load structures. Large-scale load casts penetrate up to 4.5 m into the underlying sandstone bed. Wedge-shaped load structures include metre-scale, parallel, sub-vertical features, and decimetre-scale features along the periphery of the large-scale load casts or other wedge-shaped load structures. Wedge-shaped load structures contain well-developed, medial cataclastic shear deformation bands. All load structures contain pervasive well-defined millimetre-thick to centimetre-thick internal laminae, oriented parallel to the outside form of the load structures and asymptotic to deformation bands. Both types of load structures formed because of an inverted density profile, earthquake-triggered liquefaction, and growth of irregularities (a Rayleigh-Taylor instability) on the sandstone–sandstone erosional contact. The internal laminae and deformation bands formed during deformation, and clearly demonstrate polyphase deformation, recording a transition from liquefied to hydroplastic to brittle modes of deformation. Decimetre-scale wedge-shaped load structures on the edge of the large-scale load casts probably formed towards the end of a seismic event after the sediment dewatered and increased the frictional contact of grains enough to impart strength to the sands. Metre-scale wedge-shaped load structures were created as the tips of downward foundering sediments were driven into fractures, which widened incrementally with seismic pulsation. With each widening of the fracture, gravity and a suction effect would draw additional sediment into the fracture. Superimposed laminae indicate a secondary syndeformational origin for internal laminae, probably by flow-generated shearing and vibrofluidization mechanisms. Large-scale and wedge-shaped load structures, polyphase deformation and secondary laminae may characterize soft-sediment deformation in certain fault-proximal settings. This article is protected by copyright. All rights reserved.
... Danau Bandung yang ada di Cekungan Bandung memiliki bagian yang disebut sag pond, merupakan suatu kolam relatif kecil yang pembentukannya pada daerah Bandung dipengaruhi oleh adanya Sesar Lembang (Hidayat, 2010;Hubert-Ferrari, Avsar, Ouahabi, & Lepoint, 2012;Rasmid, 2014;Simpson et al., 2014;Ghassemi et al., 2014). Sag pond pada Cekungan Bandung terletak di sebelah utara berdampingan dengan kehadiran Sesar Lembang (Hidayat, 2010). ...
Abstrak Alga merupakan mikroorganisme fotosintesis yang tidak memiliki tubuh sejati dengan distribusi lingkungan yang sangat luas dan memiliki banyak fungsi, salah satunya sebagai indikator perubahan lingkungan. Wilayah Cekungan Bandung telah dilakukan penelitian dari beberapa aspek. Akan tetapi, penelitian terutama dari aspek alga yang memperlihatkan perubahan lingkungan belum dilakukan pada daerah tersebut. Penelitian ini dilakukan untuk melihat bagaimana perubahan lingkungan dari endapan sag pond bekas Danau Bandung pada wilayah Cekungan Bandung menggunakan keberagaman alga dan ukuran butir sedimen. Metode asam dilakukan untuk memisahkan alga dari endapan, serta deskripsi batuan dilakukan untuk melihat ukuran butir endapan pada lokasi penelitian. Analisis data dan pemodelan dilakukan dengan mengelompokkan alga sesuai dengan kondisi habitatnya yang terbagi menjadi kelompok Pinnularia, Euglena, Acrinastrum dan Dinobryon. Hasil penelitian menunjukkan bahwa lokasi penelitian mengalami empat kali perubahan lingkungan yang dibagi kedalam zona-zona dan subzona. Setiap zona tersebut diawali dengan kondisi air tercemar yang ditandai oleh peningkatan kelompok Euglena dan ukuran butir halus dari sedimen. Sedangkan pada bagian akhir, perairan memperlihatkan kondisi air normal yang ditunjukkan dengan peningkatan kelompok Pinnularia dan ukuran butir sedimen yang menjadi lebih kasar.
... The Wahweap Formation, first described by Gregory and Moore (1931) as the Wahweap Sandstone, is a 360 to 460-m-thick fluvial succession comprising interbedded floodplain mudstones and channel sandstones. Peterson and Waldrop (1965) formally defined the unit as the Wahweap Formation, which was followed by more detailed sedimentological investigation by Peterson (1969), Eaton (1991), Little (1995), Pollock (1999), Simpson et al. (2008Simpson et al. ( , 2014 and Jinnah and Roberts (2011). The Wahweap Formation is exposed in southern Utah, U.S.A., along the margins of the Markagunt and Paunsaugunt plateaus (Biek et al., 2015) and extensively throughout the Kaiparowits Plateau ( Fig. 1) and is also correlated to the Masuk and Tarantula Mesa Sandstone formations to the northeast in the nearby Henry Basin (Eaton, 1990;Corbett et al., 2011;Jinnah, 2013;Lawton et al., 2014a). ...
The Western Interior of North America preserves one of the most complete successions of Upper Cretaceous marine and non-marine strata in the world; among these, the Cenomanian-Campanian units of the Kaiparowits Plateau in southern Utah are a critical archive of terrestrial environments and biotas. Here we present new radioisotopic ages for the Campanian Wahweap Formation, along with lithostratigraphic revision, to improve the geological context of its fossil biota. The widely accepted informal stratigraphic subdivisions of the Wahweap Formation on the Kaiparowits Plateau are herein formalized and named the Last Chance Creek Member, Reynolds Point Member, Coyote Point Member, and Pardner Canyon Member (formerly the lower, middle, upper, and capping sandstone members respectively). Two high-precision U-Pb zircon ages were obtained from bentonites using CA-ID-TIMS, supported by five additional bentonite and detrital zircon LA-ICP-MS ages. Improved geochronology of the Star Seep bentonite from the base of the Reynolds Point Member via CA-ID-TIMS demonstrates that this important marker horizon is over a million years older than previously thought. A Bayesian age-stratigraphic model was constructed for the Wahweap Formation using the new geochronologic data, yielding statistically robust ages and associated uncertainties that quantifiably account for potential variations in sediment accumulation rate. The new chronostratigraphic framework places the lower and upper formation boundaries at 82.17 + 1.47/−0.63 Ma and 77.29 + 0.72/−0.62 Ma, respectively, thus constraining its age to the first half of the Campanian. Additionally, a holistic review of known vertebrate fossil localities from the Wahweap Formation was conducted to better understand their spatio-temporal distribution including revised ages for early members of iconic dinosaur lineages such as Tyrannosauridae, Hadrosauridae, and Centrosaurinae. Chrono- and lithostratigraphic refinement of the Wahweap Formation and its constituent biotic assemblages establishes an important reference for addressing questions of Campanian terrestrial paleoecology and macroevolution, including dinosaur endemism and diversification throughout western North America.
In freshwater bivalves development takes place in three ways (Fig. 6.1).
1.
By producing veliger larvae as in Dreissena polymorpha, which is particularly successful wherever it first appears.
2.
By releasing fully developed young mussels from brood pouches (viviparity) as in Sphaeriidae.
3.
By passing early development as a parasitic stage on a host as in naiads (Unionoida).
Alien species (those carried outside their original ranges by human activities) have strongly affected the distribution and abundance of mollusks in many North American fresh waters. The best known of these aliens in North America is the zebra mussel (Dreissena polymorpha), which has nearly extirpated native unionid clams from infested lakes and rivers by fouling their shells and outcompeting them for food. Zebra mussels also have reduced populations of native sphaeriid clams, and both increased and reduced populations of snails. The effects of the other well-known alien bivalve in North America, Corbicula fluminea, are surprisingly poorly known. Corbicula probably caused some populations of native bivalves to decline, but other native populations seem to coexist with Corbicula. Several plausible mechanisms of interaction between Corbicula and the native biota have been proposed, but not demonstrated. Other aliens, including the recently arrived snail Potamopyrgus antipodarum, probably compete strongly with native freshwater mollusks under some circumstances. Several alien species, such as round goby and some sunfishes and crayfishes introduced outside their native ranges in North America, are effective predators on native mollusks and have strong effects on their distribution and abundance. Other aliens (particularly aquatic plants) affect mollusks by altering the food base or the physicochemical environment. Alien species can affect water quality, cycling of contaminants, and performance of biological indices of water quality. Because of ineffective control of aliens in North America, they may be an increasingly important factor in molluscan distribution as new species arrive from other continents and established species spread throughout the continent.
Historical and instrumental records of large earthquakes within the
southeastern region of the Tibetan Plateau appeared to show an irregular
and complex behaviour in the seismic activity over a relatively
short-time period. However, whether this kind of earthquake behaviour
can be extrapolated over long-time periods is not well addressed.
Herein, we present about an 8000 yr earthquake record using
palaeoseismological techniques from the Zemuhe fault (ZMHF) to assess
the earthquake behaviour and show the complexity of faulting within the
region of the Tibetan Plateau. The ZMHF along with the Anninghe (ANHF)
and Xiaojiang (XJF) faults, form the N-S-striking active fault zone at
the tectonic boundary of the southeastern margin of the Tibetan Plateau.
In the historical record, a large earthquake occurred in 1850 AD at M
7.5 along the ZMHF, with a coseismic left-lateral displacement of
3.6-4.8 m along the Daqingliangzi section where we excavated several
trenches in a small triangular-shaped sag pond. We analyse the
depositional sequences and deformation evidence in the trenches to
recreate the sequence of earthquakes that occurred from the early
Holocene until the present. From trenching and 14C ages, five
palaeoseismic events are identified and named Z through V from youngest
to oldest at 1370 AD-present, 690-940 AD, 5900-1400 BC, 6590-6350 BC and
7380-7110 BC. We associate two young events Y and Z with the historical
records of 814 AD M 7 and 1850 AD M 7.5, respectively. Moreover,
integrated palaeoearthquake results on ages of one event occurred
slightly younger than 1700-1100 BC and 1540-1270 BC from previous
palaeoseismic studies, we further constrain the age of event X at
1700-1400 BC. The average recurrence interval of earthquakes at the
Daqingliangzi section on the ZMHF is about 2300 yr. The recurrence
interval of palaeoseismic events during the early Holocene and late
Holocene is shorter than that of the middle-Holocene. The interval
between event V and W is 1030-520 yr, 2514-2214 yr between X and Y and
1036 yr between Y and Z; however, the recurrence interval between W and
X is much longer at 5190-4650 yr. Holocene surface-faulting events along
the ZMHF appear to be unevenly spaced in time. The irregular recurrence
intervals of earthquakes on the ZMHF might be related to complexity of
fault interactions in the left-lateral fault system located along the
eastern and northern boundaries of the Sichuan-Yunnan block and may
indicate regional irregularities of large earthquakes within the
southeastern region of the Tibetan Plateau.
We experimentally manipulated mussel community structure and observed mussel burrowing behavior in mesocosms held in a greenhouse. Vertical positions, vertical movements, and horizontal movements of Actinonaias ligamentina, Amblema plicata, Fusconaia flava, and Obliquaria reflexa were recorded during five 11-d trials. Community structure was manipulated by constructing communities with 11 different diversity treatments crossed with 3 different density treatments. Vertical positions, vertical movements, and horizontal movements of mussels differed significantly among diversity treatments, and vertical movements differed among density treatments. Differences among diversity treatments were caused by differences in species composition because the burrowing activity of mussels in multispecies communities could be predicted additively from single-species communities. The species used in our study vary in body size, but differences among species were still significant after accounting for body length. We think that differences in species burrowing behavior might be a result of niche partitioning of vertical space, might be a result of differing effects of temperature between species, or might be related to mechanisms to avoid dislodgement during high flows. The burrowing behavior of freshwater mussels has implications for mussel sampling protocols, the sensitivity of mussels to zebra mussel attachment, and how mussels influence benthic ecosystems.
The importance of strike-slip faulting was recognized near the turn of the century, chiefly from investigations of surficial offsets associated with major earthquakes in New Zealand, Japan, and California. Extrapolation from observed horizontal displacements during single earthquakes to more abstract concepts of long-term, slow accumulation of hundreds of kilometers of horizontal translation over geologic time, however, came almost simultaneously from several parts of the world, but only after much regional geologic mapping and synthesis.Strike-slip faults are classified either as transform faults which cut the lithosphere as plate boundaries, or as transcurrent faults which are confined to the crust. Each class of faults may be subdivided further according to their plate or intraplate tectonic function. A mechanical understanding of strike-slip faults has grown out of laboratory model studies which give a theoretical basis to relate faulting to concepts of pure shear or simple shear. Conjugate sets of strike-slip faults form in pure shear, typically across the strike of a convergent orogenic belt. Fault lengths are generally less than 100 km, and displacements along them are measurable in a few to tens of kilometers. Major strike-slip faults form in regional belts of simple shear, typically parallel to orogenic belts; indeed, recognition of the role strike-slip faults play in ancient orogenic belts is becoming increasingly commonplace as regional mapping becomes more detailed and complete. The lengths and displacements of the great strike-slip faults range in the hundreds of kilometers.The position and orientation of associated folds, local domains of extension and shortening, and related fractures and faults depend on the bending or stepping geometry of the strike-slip fault or fault zone, and thus the degree of convergent or divergent strike-slip. Elongate basins, ranging from sag ponds to rhombochasms, form as result of extension in domains of divergent strike slip such as releasing bends; pull-apart basins evolve between overstepping strike-slip faults. The arrangement of strike-slip faults which bound basins is tulip-shaped in profiles normal to strike. Elongate uplifts, ranging from pressure ridges to long, low hills or small mountain ranges, form as a result of crustal shortening in zones of convergent strike slip; they are bounded by an arrangement of strike-slip faults having the profile of a palm tree.Paleoseismic investigations imply that earthquakes occur more frequently on strike-slip faults than on intraplate normal and reverse faults. Active strike-slip faults also differ from other types of faults in that they evince fault creep, which is largely a surficial phenomenon driven by elastic loading of the crust at seismogenic depths. Creep may be steady state or episodic, pre-seismic, co-seismic, or post-seismic, depending on the constitutive properties of the fault zone and the nature of the static strain field, among a number of other factors which are incompletely understood. Recent studies have identified relations between strike-slip faults and crustal delamination at or near the seismogenic zone, giving a mechanism for regional rotation and translation of crustal slabs and flakes, but how general and widespread are these phenomena, and how the mechanisms operate that drive these detachment tectonics are questions that require additional observations, data, and modeling.Several fundamental problems remain poorly understood, including the nature of formation of en echelon folds and their relation to strike-slip faulting; the effect of mechanical stratigraphy on strike-slip-fault structural styles; the thermal and stress states along transform plate boundaries; and the discrepancy between recent geological and historical fault-slip rates relative to more rapid rates of slip determined from analyses of sea-floor magnetic anomalies. Many of the concepts and problems concerning strike-slip faults are derived from nearly a century of study of the San Andreas fault and have added much information, but solut ons to several remaining and new fundamental problems will come when more attention is focused on other, less well studied strike-slip faults.
Unionid mussels often occur as multispecies aggregates called mussel beds and in dense patches within the mussel beds themselves. Thus, their distributions are patchy at 2 spatial scales. We examined the association between mussel assemblage structure and macroinvertebrate assemblage structure at these 2 spatial scales in rivers of the Ouachita Highlands, Arkansas and Oklahoma, USA. We used multivariate variation partitioning techniques to relate variation in benthic macroinvertebrate distribution and abundance to variation in mussel assemblages, environmental variables, spatial variables, and overlapping or shared variation between these components. At the patch scale, total densities of macroinvertebrates and dominant groups (Oligochaeta, Chironomidae, Ephemeroptera, and Trichoptera) were significantly higher in patches containing mussels than where mussels were absent, and densities of macroinvertebrates were positively correlated with unionid density. In variation partitioning analyses, mussel assemblages explained almost ½ of the variation in macroinvertebrate assemblages at both spatial scales, even after removing effects of similar habitat (environmental variables) and biogeographic history (spatial variables).
The Hayward fault, a major branch of the right-lateral San Andreas fault system, traverses the densely populated eastern San Francisco Bay region, California. We conducted a paleoseismic investigation to better understand the Hayward fault's past earthquake behavior. The site is near the south end of Tyson's Lagoon, a sag pond formed in a right step of the fault in Fremont. Because the Hayward fault creeps at the surface, we identified paleoseismic events using features that we judge to be unique to ground ruptures or the result of strong ground motion, such as the presence of fault-scarp colluvial deposits and liquefaction. We correlate the most recent event evidence (E1) to the historical 1868 M 6.9 earthquake that caused liquefaction in the pond and recognize three additional paleoruptures since A.D. 1470 ± 110 yr. Event ages were estimated by chronological modeling, which incorporated historical and stratigraphic information and radiocarbon and pollen data. Modeled, mean age and 95-percentile ranges of the three earlier events are A.D. 1730 (1650-1790) yr (E2), A.D. 1630 (1530-1740) yr (E3), and A.D. 1470 (1360-1580) (E4). The ages of these paleoearthquakes yield a mean recurrence of 130 ± 40 yr. Although the mean recurrence is well determined for the period A.D. 1470-1868, individual intervals are less well determined: E1-E2, 140 +80/-70 yr; E2-E3, 100 +90/-100 yr; and E3-E4, 150 +130/-110 yr.
Sandstone intrusions are found in all sedimentary environments but have been reported most commonly from deep-water settings. They also appear to be more frequently developed in tectonically active settings where applied tectonic stresses facilitate development of high fluid pressures within the sediments. A variety of mechanisms have been cited as triggers for clastic intrusions. These include seismicity induced liquefaction, application of tectonic stresses, excess pore fluid pressures generated by deposition-related processes and the influx of an overpressured fluid from deeper within the basin into a shallow sand body. The formation of sandstone dykes and sills is investigated here by considering them as natural hydraulic fractures. When the seal on an unconsolidated, overpressured sand body fails the resulting steep hydraulic gradient may cause the sand to fluidize. The fluidized slurry can then inject along pre-existing or new fractures to form elastic intrusions. The scale and the geometry of an intrusive complex is governed by the stress state, depth and pre-existing joints or faults within the sedimentary succession, as well as the nature of the host sediments. For the simplest tectonic setting, where the maximum stress in a basin is vertical (gravitational loading), small irregular intrusions commonly result in the formation of sills at shallow depths within a few metres of the surface, whereas at greater depth dykes and sills forming elastic intrusion networks are more typical. A simple relationship is derived to calculate the maximum burial depth at which a dyke-sill complex forms as a function of the source-bed to sill height, the bulk density of the surrounding sediments, and the ratio of the vertical to horizontal effective stresses, K-0. When applied to three examples of large-scale dyke-sill complexes developed within Paleocene and Eocene deep-water reservoir sand bodies of the North Sea, maximum burial depths in a range of 375 to c. 500 m, 450-700 m and 550-850 m are estimated for intrusion of each of the three complexes.
Minimal direct evidence exists in the rock record of dinosaurs and mammals behaving as predators and prey, respectively. A newly discovered Late Cretaceous trace fossil association of digging traces of maniraptoran theropod dinosaurs and mammalian den complexes indicates a predator-prey relationship. Three distinct associated trace fossils occur within a floodplain siltstone-mudstone bed of the Upper Cretaceous Wahweap Formation in southern Utah, United States. One trace fossil morphology and its extramorphological variants record digging by a maniraptoran theropod dinosaur, possibly a dromeosaurid or troodontid. The other two are interpreted as mammalian den complexes. The proximal association of these trace fossils suggests that dinosaurs used excavation techniques to prey on mammals.
In the Bull Flat area of Grand Staircase (Escalante National Monument, Utah, United States), sedimentological analysis of the tip line area of a Late Cretaceous normal fault reveals that the fault was active during the deposition and preservation of a sag pond deposit. Sag ponds deposits are probably under-recognized in the rock record, and, as demonstrated in this study, they are helpful in chronicling activity of ancient faults. The sag pond sequence is located just below the contact between the upper and capping sandstone members of the Wahweap Formation, and it occurs within close proximity to other features of seismogenic origin. The sag pond deposit is a 2.2-m-thick succession of carbonaceous, interlaminated mudstones and siltstones intruded by sublithic sandstone seismogenic dikes and sills. At least two seismic events are required to produce the observed sequence. The initial event ruptured the surface, probably tilting the underlying strata, and produced the down-drop for the sag pond. After infi lling of the sag pond, a second seismogenic event mobilized sediment in the underlying upper member and generated the dikes and sills that intrude the sag pond deposit. Subsequently, a minor channel scoured the sag pond deposit with fl ow parallel to the fault trace. Infilling of the sag pond ended before deposition of the capping sandstone member.
Laser-fusion 40Ar/39Ar analysis of four bentonite horizons produces the first absolute ages for the 860-m-thick Kaiparowits Formation and resolves previous age uncertainty caused by ambiguous biostratigraphy. A late Campanian (Judithian) age of ca. 76.1–74.0Ma is determined, resulting in a high-resolution temporal framework for the richly fossiliferous formation. Detailed stratigraphic correlation reveals that the Kaiparowits Formation is contemporaneous with many of the most important vertebrate fossil-bearing formations in the Western Interior Basin, and with other well-studied strata across Utah and southeastern Wyoming, including portions of the Book Cliffs sequence. The Judithian age determination and correlations for the Kaiparowits Formation presented here provide a new chronological basis for addressing questions relating to mammal biostratigraphy, vertebrate evolution, biodiversity and paleobiogeography (e.g., dinosaur provincialism) in the Cretaceous Western Interior Basin.
Freshwater mussel assemblages in the Flint River Basin (FRB) of southwestern Georgia are among the most diverse in the southeastern Coastal Plain of North America. Historically, 29 species, including 7 endemics, occurred in the FRB. A drought during the summer of 2000 caused record low flows and many perennial streams dried or became intermittent. Predrought surveys conducted in 1999 allowed an assessment of the impact of the drought on mussel assemblages. During 2001, 21 stream reaches that had abundant or diverse mussel assemblages in 1999 were resurveyed. Study sites were classified as flowing or non-flowing during the drought based on data from stream gauging stations or visual observation of study reaches. Mussels were classified by conservation status, either stable, special concern, or federally endangered. Greater than 90% of the mussels observed in the lower FRB were species with stable conservation status. Special-concern species represented 5 to 6% and endangered species represented 1% of mussel abundance. Sites that ceased flowing during the drought had significant declines in the abundance of stable species and in taxa richness. Endan- gered species also showed evidence of a decline in non-flowing sites. Sites that maintained flow had increases in stable species and no change in special concern, endangered species, or species richness through the drought. Sites that showed declines in mussel abundance had a significantly lower fre- quency of wood debris than other sites. Field observations suggested that shallow depressions beneath wood debris may act as refuges for freshwater mussels during stream drying. Greatest declines in mussel abundance usually occurred in the mid-reaches of the major tributaries of the lower Flint River. These reaches depend on the Upper Floridan aquifer, which is heavily used for irrigation, to maintain base flows. Declines in mussel populations appear to be associated with unusual climatic conditions and increasing demand on the area streams and the regional aquifer system for irrigation water supply.
An extraordinary continental shell bed is reported from the Upper Cretaceous Kaiparowits Formation in southern Utah. This shell bed, referred to as the Kaiparowits Blues Ceratopsian shell bed, is highly unusual among fluvial-estuarine shell beds for its great thickness, surface area and shell density and its geometry. It covers >850 m 2 , ranges from 85 to 280 cm thick, and consists of a series of 10–50-cm-thick low-angle, dipping beds. The shells are generally undamaged and articulated (>75%), commonly with valves still closed, and strongly oriented normal to the dip orientation of the shell layers. The shell bed is interpreted as a lateral accretion bar set that developed in a point bar or midchannel bar setting. Nearly 45% of shells are encrusted by the brackish-water bryozoan Conopeum sp., indicating that deposition transpired within the upper-fluvial to mixed-fluvial–marine part of an estuarine channel system. At least five unionoid (Unionoidea) shell morphotypes are present, representative of both parautochthonous (intrachannel) and allochthonous (adjacent quiet-water pond/marsh) taxa. Taphonomic and sedimentologic investigations suggest that rapid winnowing and amalgamation of live and recently dead shells from nearby high-density mussel shoals and a smaller population of calmer-water morphotypes from surrounding floodbasin environments occurred during the waning stages of an unusually high intensity storm event, possibly a tropical storm or hurricane. A combination of extreme hydrologic events, such as catastrophic flooding, cyclonic winds, and storm surge, may have contributed to the development of the shell bed. At least 1.4 million individual unionoids are preserved in this deposit, making it one of the most voluminous and highest density fresh/brackish-water shell beds reported in the fossil or historical records.
The Wahweap Formation is an similar to 400-m-thick succession of fluvial and estuarine channel sandstones and floodbasin mudstones divided into lower, middle, upper, and capping sandstone members. Facies analysis of the Wahweap Formation on the Kaiparowits Plateau reveals the presence of ten facies associations grouped into channel and floodbasin deposits. Fades associations (FAs) from channels include: (1) single-story and (2) multistory lenticular sandstone bodies, (3) major tabular sandstone bodies, (4) gravel bedforms, (5) low-angle heterolithic cross-strata, and (10) lenticular mudrock, whereas floodbasin facies associations include: (6) minor tabular sandstone bodies, (7) lenticular interlaminated sandstone and mudrock, (8) inclined interbedded sandstone and mudrock, and (9) laterally extensive mudrock. The lower and middle members are dominated by floodbasin facies associations. The lower member consists dominantly of FA 8, interpreted as proximal floodbasin deposits including levees and pond margins, and is capped by a persistent horizon of FA 3, interpreted as amalgamated channel deposits. FAs 4 and 6 are also present in the lower member. The middle member consists dominantly of FA 9, interpreted as distal floodbasin deposits including swamp, oxbow-lake, and waterlogged-soil horizons. FAs 1, 2, 5, 6, 7, 8, and 10 are present in the middle member as well, which together are interpreted as evidence of suspended-load channels. The upper member is sandstone-dominated and consists of FAs 1, 2, 3, 5, 7, and 8. FAs 5 and 7, which occur at the base of the upper member, are interpreted as tidally influenced channels and suggest a marine incursion during deposition of the upper member. The capping sandstone is characterized by FAs 3, 4, and 6, and is interpreted to represent a major change in depositional environment, from meandering river systems in the lower three members to a low-accommodation, braided river system. Improved age constraints on the Wahweap Formation indicate that the middle and upper members were deposited during the eustatic Claggett transgression (T8 of Kauffman 1977) in the adjacent Western Interior Seaway. Additionally, facies analysis of the Wahweap Formation has revealed an increase in both the sand:mud ratio and the degree of sandstone amalgamation from the upper part of the middle member to the base of the capping sandstone, suggesting a gradual reduction in generation of accommodation in the basin. Following recent alluvial sequence stratigraphic models, the middle member is interpreted as the isolated fluvial fades tract, while the upper member represents the tidally influenced and highstand facies tracts. Maximum transgression occurred during deposition of the lowest part of the upper member, synchronous with the putatively eustatic Claggett highstand in other parts of the Western Interior Basin. The sequence boundary is placed at the base of the overlying capping sandstone member, diagnosed by a major shift in petrography and paleocurrent direction, as well as up to 4 m of fluvial incision into the underlying upper member. The capping sandstone member is interpreted as the amalgamated fluvial facies tract, and the sequence boundary at the base of the capping sandstone is regarded as tectonically induced. Improved geochronology in the formation now permits high-resolution correlation with marine units to the east and along-strike correlation with contemporaneous alluvial units up and down the western margin of the Western Interior Basin.
(Revised version accepted June 8, 1978)
ABSTRACT: Sanders, C.O. and Slemmons, D.B., 1979. Recent crustal movements in the Central Sierra Nevada—Walker Lane region of California—Nevada: Part III, the Olinghouse fault zone. In: C.A. Whitten, R. Green and B.K. Meade (Editors), Recent Crustal Movements, 1977. Tectonophysics, 52: 585-597.
The Olinghouse fault zone is one of several NE—ENE-trending fault zones and lineaments, including the Midas Trench and the Carson—Carson Sink Lineament, which exhibit left-lateral transcurrent movement conjugate to the Walker Lane in western Nevada. The active portion of this fault zone extends for approximately 23 km, from 16 km east of Reno, Nevada, to the southern extent of Pyramid Lake.
The fault can be traced for most of its length from its geomorphic expression in the hilly terrain, and it is hidden only where overlain by recent alluvial sediments. Numerous features characteristic of strike-slip faulting can be observed along the fault, including: scarps, vegetation lines, sidehill and shutter ridges, sag ponds, offset stream channels and stone stripes, enclosed rhombohedral and wedge-shaped depressions, and en-echelon fractures.
A shear zone having a maximum observable width of 1.3 km is defined principally by Riedel shears and their symmetrical P-shears, with secondary definition by deformed conjugate Riedel shears. Several continuous horizontal shears, or principal displacement shears, occupy the axial portion of the shear zone. The existence of P-shears and principal displacement shears suggests evolution of movement along the fault zone analogous to the “Post-Peak” or “Pre-Residual Structure” stage.
Historic activity (1869) has established the seismic potential of this zone. Maximum intensities and plots of the isoseismals indicate the 1869 Olinghouse earthquake had a magnitude of 6.7. Field study indicates the active length of the fault zone is at least 23 km and the maximum 1869 displacement was 3.65 m of left-slip. From maximum fault length and maximum fault displacement to earthquake magnitude relations, this corresponds to an earthquake of about magnitude 7.
This field trip consists of stops in four locations that provide insight into the San Andreas fault along the San Francisco peninsula. The first two stops provide an overview and close-up look at the fault where no urbanization has occurred. The last two stops are examples of areas where urbanization occurred directly over the fault prior to current regulations. The field trip also addresses the history of, and seismic hazard issues related to, an important part of the San Francisco Public Utilities Commission's (SFPUC) water-supply system, which is located along the San Andreas fault.
This study determined that inexpensive and easily maintained amphibians and exotic fishes could act as hosts for two species of native North American unionid mussels, and bypass the need to identify native hosts when the object is co culture mussels. Two mussel species, Lampsilis cardium and Utterbackia imbecillis, were used to parasitize 42 exotic fishes and seven potential nonpiscine host species. Nonpiscine hosts included amphibians and decapod crustaceans. Lampsilis cardium successfully metamorphosed on six species of exotic fishes, as well as on larval tiger salamanders. Utterbackia imbecillis successfully metamorphosed on 30 species of exotic fishes and all four amphibian species tested. No glochidia metamorphosed on crustaceans. Successful metamorphosis on amphibians indicates that mussel zoogeography may be more complicated than previously thought. Using surrogate hosts may be a valuable alternative to natural hosts in laboratory culture of mussels.
The reproductive biology of four mussel species (subfamily Lampsilinae), Villosa nebulosa, V. vanuxemi, Medionidus conradicus, and Lampsilis fasciola, was investigated during a 14-month study of the mussel community in Big Moccasin Creek, southwestern Virginia. Examination of histological sections of gonadal tissue from males and females of each species collected from April 1979 to May 1980 revealed that active gametogenesis proceeded throughout the year. Distinct and separate spawning periods for the four species in 1979 were as follows: M. conradicus, July 8-16; V. vanuxemi, July 25-30; V. nebulosa, August 13-20; and L. fasciola, late August. Periodic inspection of developing embryos in female marsupia indicated that glochidia of each species required 7 to 8 weeks to develop after fertilization. Mature glochidia were readily identified to genus but differentiation of V. nebulosa from V. vanuxemi was difficult. Drift nets (130-µm mesh) set downstream from the mussel community captured glochidia of M. conradicus throughout the year except July and August; glochidia of V. vanuxemi from October to May; and glochidia of V. nebulosa from April to early August. Females of L. fasciola discharged glochidia from May to mid-August.
To determine the potentiality for dispersal of Asiatic clams (Corbicula manilensis) via the intestinal tract of waterfowl, live clam specimens were force-fed to adult male lesser scaup ducks (Aythya affinis). Examination of duck excreta revealed no egestion of viable individuals. In a second experiment, a simulated waterfowl gizzard was utilized to determine the chemical and physical effects of arian digestion on live Asiatic clams. Mortality of test clams was 100% after 1 min exposure in a solution of synthetic gastric juice.
The association of vertebrate, invertebrate, plant, and ichnofossils from two Upper Cretaceous lacustrine environments in West Texas permits interpretation of the paleolimnology of part of the Javelina Formation. The relatively rich fossil assemblages permit studies of paleoenvironments and food web complexities, as well as provide paleohydrological information. In Big Bend National Park, at least three juvenile Alamosaurus sauropods were preserved in situ in anoxic mud among shallow-water lacustrine charophytes. Alamosaur remains in the Moon Valley section are both in situ and transported. Mussels, gastropods, gar, and crayfish trace fossils largely define the paleoecology, as well as reveal details of specific biological relationships, especially at several intervals in Moon Valley. The modern larval-host reproduction system of mussels may have existed in the possible case of an ectoparasitic relationship in the co-occurrence of fossil Unio and Lepisosteus. Relatively fine time-frame resolution is present based on investigations of the mussel, Unio, and the presence of crayfish burrows. Unio sclerochronology indicates continuous inundation for at least seven years at two different stratigraphic intervals. Horizons of fossil crayfish burrows record both periods of nondeposition and the level of the water table at those times. Viviparus gastropods suggest subtleties of water movement and oxygen concentration. Studies of the dinosaur bones and the presence of probable footprint impressions suggest dinosaurs visited these types of water bodies.
Liquefaction features can be used in many field settings to estimate the recurrence interval and magnitude of strong earthquakes through much of the Holocene. These features include dikes, craters, vented sand, sills, and laterally spreading landslides. The relatively high seismic shaking level required for their formation makes them particularly valuable as records of strong paleo-earthquakes. This state-of-the-art summary for using liquefaction-induced features for paleoseismic interpretation and analysis takes into account both geological and geotechnical engineering perspectives.
Late Holocene marsh deposits composing a terrace about 55 km northeast of Los Angeles, California, contain geologic evidence of many large seismic events produced by slip on the San Andreas fault since the sixth centrury A.D. I excavated several trenches into the deposits in order to study this evidence. The principal indicators of past events are (1) sandblows and other effects of liquefaction, (2) the termination of secondary faults at distinct levels within the stratigraphic section, and (3) sedimentary deposits and faulted relationships along the main fault. The effects upon the marsh deposits of six of the eight prehistoric events are comparable to those of the great (Ms=81/4+) 1857 event, which is the youngest of the nine events disturbing the strata and is associated with about 41/2 m of right lateral slip nearby. Two large events may be smaller than this. Radiocarbon dates indicate that the events occurred in the nineteenth, eighteenth, fifteenth, thirteenth, late twelfth, tenth, ninth, seventh, and sixth centuries A.D. Recurrence intervals average 160 years but vary from 1/2 century to about 3 centuries. The dates may indicate a fairly systematic pattern of occurrence of large earthquakes.
Unionid mussels are among the largest and longest-lived invertebrates in shallow benthic communities, where they often make up most of the biomass. However, they spend their juvenile stages and extended periods of their adult lives buried in the sediments and emerge from the sediments only during part of the year. I tested whether the magnitude and timing of emergence of Elliptio complanata, an abundant native mussel, are related to water temperature, upwelling activity, site exposure, and sediment characteristics. Seasonal changes in the density and average body size of mussels on the sediment surface were measured at 26 shallow (2-m) littoral sites in a multibasin lake to determine which factors are most useful in predicting their emergence. In general, densities started increasing in early spring, peaked in spring or sometime in summer, and declined through autumn, but the magnitude (ratio of minimum-to-maximum surface density) and timing (date of maximum surface density, start of autumn decline) of emergence differed between sites. Maximum mussel surface density was on average (median) 2× higher than minimum surface density and was higher at exposed sites with fine sediment particles than at more protected sites. The timing of mussel emergence and burial were most closely related to site exposure. Mussels reached their maximum surface density earlier, but they also started burying earlier at the most exposed sites (effective fetch >900 m). At more protected sites, maximum densities were reached later in summer, after the main period of reproduction, and densities did not start declining until late summer. The only exceptions were 2 protected sites with deep, fine, highly organic and, presumably, anoxic sediments, where mussels emerged early in the spring and did not bury until late in the season. The timing of mussel emergence varied greatly in different parts of the littoral zone. These results suggest that the functional role of unionid mussels in shallow benthic communities might change seasonally and might change differently at sites exposed to different levels of physical disturbance.
Growth faults and synorogenic sedimentary strata preserved in Upper Cretaceous units on the margin of the Kaiparowits Basin in southern Utah pinpoint the timing of onset of the Laramide orogeny in this region between 80 and 76 Ma. The newly identified listric normal faults, exposed in the steep limb of the East Kaibab monocline, sole into shales and evaporites of the Jurassic Carmel Formation. Faults lose displacement up-section through the Cretaceous Wahweap Formation and are associated with numerous coseismic sedimentary features. Fault orientations and slip vectors yield strain directions consistent with fold-related extension parallel to the axis of the growing East Kaibab monocline, or with development of a pull-apart basin at a bend in the trend of the fold. The association of the faults with the steep limb of a major basement-cored structure links them to initial Laramide movement along the Kaibab Uplift. When combined with recent radiometric ages of rock units bracketing the fault-induced growth strata, these sedimentary and structural features narrowly define the onset of Laramide deformation in the western Colorado Plateau.
Sand dikes and sills in glaciomarine sediments record two liquefaction events in Newbury, Massachusetts, the meizoseismal area of the A.D. 1727, felt-area magnitude 5.0, earthquake. During the 1727 earthquake, Newbury experienced ground shaking on the order of modified Mercalli intensity VII and underwent ground failure typical of liquefaction. According to accounts of the earthquake, ground cracks with separations of up to 0.6 m formed; sand and water vented through at least ten ground cracks; land was locally elevated; and firm ground was changed to quagmire. Crosscutting relations and radiocarbon dating of woody material associated with liquefaction features exposed by trenching indicate that at least two moderate to large earthquakes have occurred in the Newbury area in the past 4 ka. The relatively unweathered, younger features are consistent in style and location with ground failure described for the 1727 earthquake. The more weathered, older features probably formed during a prehistoric earthquake. Glaciomarine and other deposits that may be prone to liquefaction are widely distributed in northeastern North America; they provide the opportunity to develop criteria for identifying paleoliquefaction events and to better determine earthquake hazard in the region.
We identify and describe a series of east-west left-lateral strike-slip faults (named the Songino-Margats, the Hag Nuur, the Uliastay and the South Hangay fault systems) in the Hangay mountains of central Mongolia: an area that has little in the way of recorded seismicity and which is often considered as a rigid block within the India-Eurasia collision zone. The strike-slip faults of central Mongolia constitute a previously unrecognized hazard in this part of Mongolia. Each of the strike-slip faults show indications of late Quaternary activity in the form of aligned sequences of sag-ponds and pressure-ridges developed in alluvial deposits. Total bed-rock displacements of ~3 km are measured on both the Songino-Margats and South Hangay fault systems. Bed-rock displacements of 11 km are observed across the Hag Nuur fault. Cumulative offset across the Uliastay fault systems are unknown but are unlikely to be large. We have no quantitative constraint on the age of faulting in the Hangay. The
A sequence of peat beds was deposited in a small sag pond along the Cucamonga fault zone during late Holocene. The stratigraphic section of this deposit is described; several beds have been radiocarbon dated. Comparison of radiometric and stratigraphic chronologies allows reconstruction of the geomorphic history of the sag pond, which extends back 3000 years, and establishes evidence that the fault has been active within the very recent geologic past.
The accuracy of the thermoluminescence (TL) method for dating Holocene paleoearthquakes was tested on fine-grained sediments from three trenches excavated across the American Fork segment of the Wasatch fault zone, Utah. Sediments exposed in the trenches targeted for TL dating include A horizons of buried soils, loessial silts, and sag pond muds. This study indicates that these sediments were fully light-bleached prior to deposition and are suitable for TL dating. Equivalent dose estimates by the regeneration, total- and partial-bleach methods are in general agreement, and mean TL age estimates are concordant with radiocarbon ages. Combined radiocarbon and TL age estimates indicate three major episodes of faulting at 5.3±0.3, 2.6±0.3 and 0.5±0.2 kyr ago.
As unionids can become dislodged with high flows, it may prove beneficial for an individual to minimize its exposure to the flow. This can be accomplished by either burrowing as deep as possible or orienting in a way that effectively reduces the drag exerted on the mussel by the flow. The patterns of orientation were examined in unionids with respect to hydrological variability. The orientation of mussels to flow was measured at four sites along an event river (hydro- logically variable) and a stable river (hydrologically stable). Burrowing depth was measured at a refer- ence site in each river. Most individuals in both river types were oriented with their siphons pointing upstream. Mussel orientation differed significantly between the two river types with mussels in the event river orienting more parallel to the flow than those in the stable river. Mussels in the event river were signif- icantly larger than those in the stable river but the size of a mussel did not determine its orientation within a river. Burrowing depth did not differ for mussels between event and stable rivers. The observed differ- ences in orientations among river types are likely a function of differences in the pattern of orientation of the mussel community as a whole, within each drain- age. This burrowing behaviour may be an attribute that enhances the adaptations mussels have for remaining burrowed in the sediment.
Petrographic and dispersal data are essential to correct interpretation of mechanisms that create continental sequence-stratigraphic architecture. A case study from southern Utah demonstrates that Upper Cretaceous (upper Santonian-Campanian) alluvial successions in the southernmost part of the Cordilleran foreland basin were deposited by fluvial systems of contrasting drainage directions and provenance, and suggests that different mechanisms governed their sequence architecture. Most of the rivers flowed northeast, subparallel to the basin foredeep. Less common fluvial systems flowed to the east-southeast. The fluvial sandstones fall naturally into four petrofacies: (1) quartzofeldspatholithic (mean Qt(61)F(19)L(20)); (2) feldspatholithic (Qt(29)F(19)L(52)); (3) quartzolithic (Qt(75)F(6)L(20)); and (4) quartzose (Qt(99)F(1)L(1)). Petrofacies 1 and 2 were derived from mixed supracrustal and basement sources to the southwest and south, respectively, whereas petrofacies 3 and 4 were derived from uplifted thrust sheets of the Sevier orogenic belt to the southwest and west, respectively. Only the east-southeast-flowing rivers transported the quartzose petrofacies. The fluvial strata, which include the uppermost Straight Cliffs, Wah-weap, and Kaiparowits formations, form two large-scale stratigraphic successions typically interpreted as continental stratigraphic sequences hundreds of meters thick. Each succession begins with an amalgamated braided-fluvial deposit, grades to mudstone-rich strata with low sandstone-body connectivity, and culminates in highly connected sandstone bodies with multistory stacking. The basal amalgamated deposits of each succession are architecturally similar, but their compositional and dispersal characteristics are different. Quartzofeldspatholithic, quartzolithic, and quartzose sandstones above the lower base-level shift are variable, but generally similar in compositional and dispersal characteristics to both underlying and overlying strata, a phenomenon termed here congruence. In contrast, quartzose amalgamated fluvial sandstone above the upper base-level shift differs sharply in composition and dispersal direction from underlying and overlying lithic-rich strata. The foredeep aids controlled the progradation direction of the congruent shift, which was likely driven by climatically induced sediment influx, a eustatic fall, or both. In the case of the incongruent shift, increased sediment supply permitted the rivers to cross the foredeep. Temporal association of the upper amalgamated deposit with active structures in the thrust belt and foreland basin indicates that syntectonic thrust uplift, not isostatic uplift or climate, caused the influx of quartz.
Paleoseismic investigations of the Lavic Lake fault at Lavic Lake playa place constraints on the timing of a possible earlier earthquake along the 1999 Hector Mine rupture trace and reveal evidence of the timing of the penultimate earthquake on a strand of the Lavic Lake fault that did not rupture in 1999. Three of our four trenches, trenches A, B, and C, were excavated across the 1999 Hector Mine rupture; a fourth trench, D, was excavated across a vegetation lineament that had only minor slip at its southern end in 1999. Trenches A–C exposed strata that are broken only by the 1999 rupture; trench D exposed horizontal bedding that is locally warped and offset by faults. Stratigraphic evidence for the timing of an earlier earthquake along the 1999 rupture across Lavic Lake playa was not exposed. Thus, an earlier event, if there was one along that rupture trace, predates the lowest stratigraphic level exposed in our trenches. Radiocarbon dating of strata near the bottom of trenches constrains a possible earlier event to some time earlier than about 4950 B.C.
Buried faults revealed in trench D are below a vegetation lineament at the ground surface. A depositional contact about 80 cm below the ground surface acts as the upward termination of fault breaks in trench D. Thus, this contact may be the event horizon for a surface-rupturing earthquake prior to 1999—the penultimate earthquake on the Lavic Lake fault. Radiocarbon ages of detrital charcoal samples from immediately below the event horizon indicate that the earthquake associated with the faulting occurred later than A.D. 260. An approximately 1300-year age difference between two samples at about the same stratigraphic level below the event horizon suggests the potential for a long residence time of detrital charcoal in the area. Coupled with a lack of bioturbation that could introduce young organic material into the stratigraphic section, the charcoal ages provide only a maximum bounding age; thus, the recognized event may be younger.
There is abundant, subtle evidence for pre-1999 activity of the Lavic Lake fault in the playa area, even though the fault was not mapped near the playa prior to the Hector Mine earthquake. The most notable indicators for long-term presence of the fault are pronounced, persistent vegetation lineaments and uplifted basalt exposures. Primary and secondary slip occurred in 1999 on two southern vegetation lineaments, and minor slip locally formed on a northern lineament; trench exposures across the northern vegetation lineament revealed the post-A.D. 260 earthquake, and a geomorphic trough extends northward into alluvial fan deposits in line with this lineament. The presence of two basalt exposures in Lavic Lake playa indicates the presence of persistent compressional steps and uplift along the fault. Fault-line scarps are additional geomorphic markers of repeated slip events in basalt exposures.
The burrowing behaviour and rates of burrowing on sand, clay, mud, and gravel were determined for three species of freshwater mussels, Lampsilis radiata (Barnes), Anodonta grandis Say, and Elliptio complanata (Solander). Burrowing is achieved by a series of probing and digging movements by the foot, alternating with adduction or closing of the valves, and foot contraction, which cause the valves to be forced downwards into the substratum. Overall burrowing abilities were superior in sand in comparison to gravel in all three species, and E. complanata burrowed more rapidly than the other species in clay, sand, and gravel. Righting was not accomplished in soft liquid mud but burrowing was achieved by frequent, rapid adduction of the valves. Although righting by both E. complanata and A. grandis in a clay substratum was slower than in either sand or gravel, both species burrowed more rapidly in clay. Although there were differences in burrowing abilities between species and in the four substrata tested, the ultimate success in burrowing by all three species suggests that substrate is not the direct causal factor affecting local distribution of freshwater mussels.
Mean seasonal species composition of living molluscan communities was compared with the composition of current dead assemblages in the sediments of three sites located in the Delta Marsh of southern Lake Manitoba. Dead shells were more numerous in vegetated than in bare areas, resulting primarily from the affinities of living molluscs for vegetated areas. Redistribution patterns of empty shells were not significantly different for vegetated and bare areas, as judged from distributions of passively transported land shells in the sediments. Significant differences were observed at all sites between species frequencies in living and corresponding dead assemblages averaged for the season. Proportions of living to dead individuals per unit bottom area indicated higher attrition rates with increasing energy conditions as well as with increasing shell size. Differential attrition may result in overrepresentation of small species in fossil assemblages.