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Stratigraphy of the Mesaverde Group in the central and eastern greater Green River basin, Wyoming, Colorado, and Utah
This paper establishes a stratigraphic framework for the Mesaverde Group, nearly 5,000 ft thick, in the central and eastern greater Green River basin based on data from measured outcrop sections and drill holes. Stratigraphic correlations are supported by ammonite zonation. No new stratigraphic names are introduced, and no nomenclature problems are discussed. Five long measured sections through the Mesaverde Group are described. The lower part of the Mesaverde Group, comprising the Rock Springs, Blair, Haystack Mountains, Allen Ridge, and Iles Formations, was deposited during a major eastward regression of the interior Cretaceous seaway of North America during the late Santonian and early Campanian. This regression was followed by regional uplift of the central Rocky Mountain area during the middle Campanian. The regional uplift was accompanied by widespread nondeposition and erosion, which, in turn, were followed by deposition of the Ericson and Pine Ridge Sandstones. The upper part of the Mesaverde Group, comprising the Almond and Williams Fork Formations, was deposited during a major westward transgression of the interior seaway in the early Maestrichtian. The major marine transgressions and regressions of the interior seaway were caused by eustatic changes of sea level, whereas intervening periods of nondeposition and erosion resulted from tectonism in the Sevier orogenic belt west of the study area. Formations of the Mesaverde Group are composed of sediments deposited in a landward-seaward progression of alluvial-plain, floodplain, coastal-plain, barrier-plain, tidal-flat, delta-plain, marine-shoreline, and marine-shelf and slope depositional environments. Each of these depositional environments is represented by specific lithofacies, sedimentary structures, and fossils, which are characteristic of depositional settings determined by water salinity, water depth, sedimentary and diagenetic processes, and the nature of sediment source terranes. The Mesaverde Group was deposited mainly along the western margins of the interior Cretaceous seaway as marine shorelines that trended north to northeast across the study area. Arcuate deltas, which formed at the mouths of major rivers along these shorelines, spread eastward onto shallow marine shelves. Embayed shoreline areas between the deltas were the sites of barrier-island and tidal-flat deposition. Alluvial-plain, flood-plain, and coastal-plain environments were present inland. The marine shorelines were tidally influenced and wave dominated, and shoreline deposits were mostly thick, linear sheets of quartzose sandstone. Deposition was largely controlled by the emergence or submergence of shoreline areas. Stillstands occurred close to local transgressions and regressions, depending on rates of sedimentation and subsidence.
Stratigraphy of the Cretaceous Hornbrook Formation, southern Oregon and Northern California
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Stratigraphy of the Upper Cretaceous Fox Hills Sandstone and Adjacent Parts of the Lewis Shale and Lance Formation, East Flank of the Rock Springs Uplift, Southwest Wyoming
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Stratigraphy, structure, and graptolites of an Ordovician and Silurian sequence in the Terra Cotta Mountains, Alaska Range, Alaska
The geology and graptolite faunas of the Terra Cotta Mountains in south-central Alaska are described. Three new formations of Paleozoic age are named and described; they include graptolitiferous argillaceous rocks, sandstones, and limestones of the Dillinger Terrane. The structure and tectonics of the mapped area arc also discussed. Ninety-five species of Ordovician and Silurian graptolites, including four new species, are described and illustrated. The faunal succession is divided into nine Ordovician and ten Silurian biostratigraphic zones and is correlated with graptolite faunas found elsewhere.
Lithofacies, depositional environments, and regional stratigraphy of the lower Eocene Ghazij Formation, Balochistan, Pakistan
The coal-bearing, lower Eocene Ghazij Formation is exposed intermittently over a distance of 750 kilometers along the western margin of the Axial Belt in north-central Pakistan. Underlying the formation are Jurassic to Paleocene carbonates that were deposited on a marine shelf along the pre- and post-rift northern margin of the Indian subcontinent. Overlying the formation are middle Eocene to Miocene marine and nonmarine deposits capped by Pliocene to Pleistocene collision molasse. The lower part of the Ghazij comprises mostly dark gray calcareous mudrock containing foraminifers and rare tabular to lenticular bodies of very fine grained to finegrained calcareous sandstone. We interpret the lower portion of this part of the Ghazij as outer-shelf deposits, and the upper portion as prodelta deposits. The middle part of the formation conformably overlies the lower part. It comprises medium-gray calcareous mudrock containing nonmarine bivalves, fine- to medium-grained calcareous sandstone, and rare intervals of carbonaceous shale and coal. Sandstone bodies in the middle part, in ascending stratigraphic order, are classified as Type I (coarsening-upward grain size, contain the trace fossil Ophiomorpha, and are commonly overlain by carbonaceous shale or coal), Type II (mixed grain size, display wedge-planar cross stratification, and contain fossil oyster shells and Ophiomorpha), and Type III (finingupward grain size, lenticular shape, erosional bases, and display trough cross stratification). These three types of bodies represent shoreface deposits, tidal channels, and fluvial channels, respectively. Mudrock intervals in the lower portion of this part of the formation contain fossil plant debris and represent estuarine deposits, and mudrock intervals in the upper portion contain fossil root traces and represent overbank deposits. We interpret the middle part of the Ghazij as a lower delta plain sequence. Overlying the middle part of the Ghazij, possibly unconformably, is the upper part of the formation, which comprises calcareous, nonfossiliferous, light-gray, brown, and red-banded mudrock, and rare Type III sandstone bodies. Much of the mudrock in this part of the formation represents multiple paleosol horizons. Locally, a limestone-pebble conglomerate is present in the upper part of the formation, either at the base or occupying most of the sequence. We interpret all but the uppermost portion of the upper part of the Ghazij as an upper delta plain deposit. Thin sections of Ghazij sandstones show mostly fragments of limestone, and heavy-liquid separations reveal the presence of chromite. Paleocurrent data and other evidence indicate a northwestern source area. During earliest Eocene time, the outer edge of the marine shelf off the Indian subcontinent collided with a terrestrial fragment positioned adjacent to, but detached from, the Asian mainland. This collision caused distal carbonateplatform deposits to be uplifted, and an intervening intracratonic sea, the Indus Foreland Basin, was created. Thus for the first time, the depositional slope switched from northwest facing to southeast facing, and a northwestern source for detritus was provided. We conclude that the Ghazij was deposited as a prograding clastic wedge along the northwestern shore of this sea, and that the formation contains sedimentologic evidence of a collisional event that predates the main impact between India and Asia.
Stratigraphy and depositional environments of middle Proterozoic rocks, northern part of the Lemhi Range, Lemhi County, Idaho
Geologic mapping shows Middle Proterozoic strata form two packages of strata north and south of the Tertiary Lem Peak normal fault. Strata north of the fault, the main focus of this report, contain thicknesses and lithofacies of the Big Creek and Apple Creek formations of the Lemhi Group that contrast with those south of the fault. Strata south of the fault, examined mainly in reconnaissance, may have been thrust eastward prior to being downdropped.
Stratigraphy and depositional environments of sediments from five cores from Screven and Burke Counties, Georgia
No abstract available.
Quaternary paleoseismology and stratigraphy of the Yucca Mountain area, Nevada
No abstract available.
Stratigraphy, lithology, and sedimentary features of Quaternary alluvial deposits of the South Platte River and some of its tributaries east of the Front Range, Colorado
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Sedimentology and sequence stratigraphy of the Cretaceous Nanushuk, Seabee, and Tuluvak formations exposed on Umiat Mountain, north-central Alaska
Upper Cretaceous strata of the upper part of the Nanushuk Formation, the Seabee Formation, and the lower part of the Tuluvak Formation are exposed along the Colville River on the east flank of Umiat Mountain in north-central Alaska. The Ninuluk sandstone, which is the uppermost unit of the Nanushuk Formation, displays a vertical succession of facies indicative of deposition in an upward-deepening estuarine through shoreface setting. A marine-flooding surface lies between the Ninuluk sandstone and organic-rich shale of the basal part of the Seabee Formation. The Ninuluk sandstone and the lower part of the Seabee Formation are interpreted as components of a transgressive-systems tract. The lowest, well-exposed strata in the Seabee Formation are a succession of shoreface sandstone beds in the middle of the formation. Integration of outcrop information and the Umiat No. 11 well log suggests that this sandstone succession rests on a sequence boundary and is capped by a marine-flooding surface. The sandstone succession is interpreted as a lowstand-systems tract. The upper part of the Seabee Formation includes a thick interval of organic-rich shale deposited in a dysaerobic offshore environment, and the gradational Seabee-Tuluvak contact is a coarsening-upward shale-to-sandstone succession deposited in a prodelta/delta-front environment. The observation that the upper part of the Seabee Formation correlates with seismic clinoforms suggests that dysaerobic conditions extended well up onto the prodelta slope during intervals of transgression and highstand. Correlation of the Umiat Mountain outcrop section with well logs and seismic data suggests that sequence boundaries and lowstand shoreface deposits may be common in the Seabee Formation and that wave action may have been important in transporting sand to the paleoshelf margin. These conclusions may contribute to an enhanced understanding of sand distribution in prospective lowstand turbidite deposits in the subsurface of the central North Slope of Alaska.
Age, stratigraphy, and correlations of the late Neogene Purisima Formation, central California coast ranges
The Purisima Formation is an important upper Miocene and Pliocene stratigraphic unit in central California, cropping out from the coast at Point Reyes north of San Francisco to more extensive exposures in the Santa Cruz Mountains to the south. The fine-grained rocks in the lower parts of the Purisima Formation record a latest Miocene transgressive event, whereas the middle and upper parts of the formation consist of increasingly clastic-rich siltstones and sandstones resulting from uplift of adjacent coastal regions and the Sierra Nevada during Pliocene transgressive and regressive sea-level events. Exposures of the Purisima occur in three different, fault-bounded, structural blocks - the Santa Cruz, Pigeon Point, and Point Reyes tectonic blocks - that complicate correlations and regional age assignments. We summarize and compare published and new biostratigraphic and geochronologic data for various exposures of the Purisima Formation on the basis of mollusks, diatoms, radiometric dating, magnetostratigraphy, tephrochronology, and strontium isotope dating. On the basis of these data, we conclude that the Purisima Formation ranges in age from the latest Miocene (about 7 Ma) to the late Pliocene (about 2.6 Ma). The Purisima Formation of Santa Cruz County, exposed in the sea cliffs from Santa Cruz to Rio del Mar, is here designated a supplementary reference section because it is the most complete and well studied Purisima section in central California.
Stratigraphy and facies of Cretaceous Schrader Bluff and Prince Creek Formations in Colville River Bluffs, North Slope, Alaska
Stratigraphic and sedimentologic studies of facies of the Upper Cretaceous rocks along the Colville River Bluffs in the west-central North Slope of Alaska identified barrier shoreface deposits consisting of vertically stacked, coarsening-upward parasequences in the Schrader Bluff Formation. This vertical stack of parasequence deposits represents progradational sequences that were affected by shoaling and deepening cycles caused by fluctuations of sea level. Further, the vertical stack may have served to stabilize accumulation of voluminous coal deposits in the Prince Creek Formation, which formed braided, high-sinuosity meandering, anastomosed, and low-sinuosity meandering fluvial channels and related flood plain deposits. The erosional contact at the top of the uppermost coarsening-upward sequence, however, suggests a significant drop of base level (relative sea level) that permitted a semiregional subaerial unconformity to develop at the contact between the Schrader Bluff and Prince Creek Formations. This drop of relative sea level may have been followed by a relative sea-level rise to accommodate coal deposition directly above the unconformity. This rise was followed by a second drop of relative sea level, with formation of incised valley topography as much as 75 ft deep and an equivalent surface of a major marine erosion or mass wasting, or both, either of which can be traced from the Colville River Bluffs basinward to the subsurface in the west-central North Slope. The Prince Creek fluvial deposits represent late Campanian to late Maastrichtian depositional environments that were affected by these base level changes influenced by tectonism, basin subsidence, and sea-level fluctuations.
Stratigraphy and depositional environments of the upper Pleistocene Chemehuevi Formation along the lower Colorado River
The Chemehuevi Formation forms a conspicuous, widespread, and correlative set of nonmarine sediments lining the valleys of the Colorado River and several of its larger tributaries in the Basin and Range geologic province. These sediments have been examined by geologists since J. S. Newberry visited the region in 1857 and are widely cited in the geologic literature; however their origin remains unresolved and their stratigraphic context has been confused by inconsistent nomenclature and by conflicting interpretations of their origin. This is one of the most prominent stratigraphic units along the river below the Grand Canyon, and the formation records an important event or set of events in the history of the Colorado River. Here we summarize what is known about these deposits throughout their range, present new stratigraphic, sedimentologic, topographic, and tephrochronologic data, and formally define them as a lithostratigraphic unit. The Chemehuevi Formation consists primarily of a bluff-forming mud facies, consisting of gypsum-bearing, horizontally bedded sand, silt, and clay, and a slope-forming sand facies containing poorly bedded, well sorted, quartz rich sand and scattered gravel. The sedimentary characteristics and fossil assemblages of the two facies types suggest that they were deposited in flood plain and channel environments, respectively. In addition to these two primary facies, we identify three other mappable facies in the formation: a thick-bedded rhythmite facies, now drowned by Lake Mead; a valley-margin facies containing abundant locally derived sediment; and several tributary facies consisting of mixed fluvial and lacustrine deposits in the lower parts of major tributary valleys. Observations from the subsurface and at outcrops near the elevation of the modern flood plain suggest that the formation also contains a regional basal gravel member. Surveys of numerous outcrops using high-precision GPS demonstrate that although the sand facies commonly overlies the mud facies where the two are found together, contacts between the two occur over a range in elevation, and as a consequence, the sand and mud facies are similarly distributed both horizontally and vertically throughout the valley. Collectively, the outcrops of the formation lie below a smooth elevation envelope that slopes 50 percent more steeply than the historic (pre-Hoover Dam) valley, from nearly 150 m above the historic flood plain near the mouth of the Grand Canyon to less than 30 m above the flood plain at the head of the flood plain near Yuma, Arizona. The steepness of the valley at the peak of aggradation probably represents a depositional slope. Layers of fine grained volcanic tephra have been found below and within the Chemehuevi Formation at five widely separated sites, one of which is now submerged beneath Lake Mead. Major element geochemistry of glass shards from the four accessible tephra sites were analyzed. Three of the sampled tephra layers are interbedded within the Chemehuevi Formation, and a fourth tephra conformably underlies the formation. The three interbedded tephra layers are similar enough to one another that they are probably from the same eruptive unit, hereafter referred to as the Monkey Rock tephra bed. The other sample, which locally underlies the formation, is similar enough to the Monkey Rock tephra bed to suggest it is from the same volcanic source area; however, it may not be from the same eruption, and thus may not be the same age. On the basis of the stratigraphic contexts of chemically similar tephra layers found elsewhere in the Basin and Range, we suspect that the source area is the Mammoth Mountain dome complex in Long Valley, east-central California. Two samples of proximal Mammoth Mountain pumice were analyzed and produced geochemical signatures similar to all four of the Chemehuevi Formation tephra, supporting Mammoth Mountain as a possible source area. The Mammoth Mountain volcanic center produced eruptions between about 111±2 and 57±2 ka and was most active in the later part of this time interval, during Marine Oxygen Isotope (MOI) stage 4 (between 74 and 59 ka ago). Chemically similar tephra in cores from Owens Lake and Walker Lake are approximately 70 and 74 ky old, based on age models of those cores. Other lines of stratigraphic evidence from nine tephra-containing sections in the Basin and Range are also consistent with an age assignment for the Monkey Rock tephra of ~72 ky, near the beginning of MOI stage 4. We propose to designate the Chemehuevi Formation as a formal lithostratigraphic unit, and propose as the type section a well exposed outcrop near the ranger station at Katherine Landing, Arizona, in the Lake Mead National Recreation Area. This exposure shows the two dominant facies, an example of one of the four known tephra layers, and interbedded lenses of locally derived gravel. In the type section, as in many of the other examples of the formation, the sand facies overlies the mud facies on a conspicuous, abrupt erosional surface; however, nearby is a contiguous section demonstrating that the mud and sand facies interfinger. In addition to the type section, measured reference sections compiled here illustrate other important lithologic and stratigraphic features of the formation. Our preferred interpretation of the Chemehuevi Formation is that it contains the remnants of deposits formed during a single major episode of fluvial aggradation, during which the Colorado River filled its valley with a great volume of dominantly sand-size sediment. This would reflect an increase in the supply of sand-size sediment, and(or) a reduction in transport capacity below the mouth of Grand Canyon. The most likely cause for the aggradation is an extraordinary increase in sand supply, likely due to widespread climatic change. However, other explanations have not been ruled out. Other aggradation events predated the Chemehuevi Formation, and some smaller events may have postdated the formation. However, the Chemehuevi Formation contains the remnants of the most recent large magnitude (>100 m) aggradation of the Colorado River.
The stratigraphy of the Upper Cretaceous rocks north of the Arkansas River in eastern Colorado
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Cambrian and Ordovician stratigraphy, conodont biostratigraphy, and microfacies analysis to support 1:24,000-scale geologic mapping of the southern Lake Champlain valley, New York and Vermont
Introduction Geologic mapping in the southern Lake Champlain valley of New York and Vermont (fig. 1) has required evaluation of stratigraphic nomenclature used since the early 1900s. The paleogeography of the Cambrian and Ordovician adjacent to the Adirondack Highlands, the high-relief mountains of Proterozoic igneous and metamorphic rocks of northern New York, has impacted the lithostratigraphic succession, facies changes, and several hiatuses that occur within the package of clastic and carbonate rocks that demonstrates overall deepening consistent from the southern to northern Appalachians. The stratigraphic nomenclature used for the geologic mapping of the Cambrian and Ordovician strata of the study area is derived from evaluation of names put forth by previous workers from areas of northern New York and western Vermont. Descriptions of type localities or type sections and other criteria set forth by the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature [NACSN], 2021) were consulted. The North American Stratigraphic Code states that lithostratigraphic units, such as the fundamental unit, the formation, be defined by lithic character and be mappable (NACSN, 2021, Article 24). However, some early workers (Walcott, 1912; Kay, 1937; Fisher and Hanson, 1951; Oxley and Kay, 1959) have defined stratigraphic units based on fossil content, which may or may not be facies dependent, and hence may not be mappable as lithostratigraphic units. Although preservation of older names takes priority in stratigraphic nomenclature (NACSN, 2021, Article 7), some do not have adequately defined stratotypes or are located a distance away with different lithologies than that in the study area. Biostratigraphy can be a helpful tool for understanding correlations and facies changes. Herein, conodont biostratigraphy is utilized to date and correlate separate lithologic units. Further, microfacies analyses of units through inspection of petrographic thin sections provide paleogeographic information that helps in understanding the genesis of geologic units.
Geology of the Kettleman Hills oil field, California: Stratigraphy, paleontology, and structure
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Mollusca from the Miocene and lower Pliocene of Virginia and North Carolina, Part 1, Pelecypoda, with a summary of the stratigraphy
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