Ecology, paleontology, and stratigraphy
Understanding of the habits of living organisms aids interpretations of fossiliferous sediments.
Science↗
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Hafnium isotope stratigraphy of ferromanganese crusts
A Cenozoic record of hafnium isotopic compositions of central Pacific deep water has been obtained from two ferromanganese crusts. The crusts are separated by more than 3000 kilometers but display similar secular variations. Significant fluctuations in hafnium isotopic composition occurred in the Eocene and Oligocene, possibly related to direct advection from the Indian and Atlantic oceans. Hafnium isotopic compositions have remained approximately uniform for the past 20 million years, probably reflecting increased isolation of the central Pacific. The mechanisms responsible for the increase in 87Sr/86Sr in seawater through the Cenozoic apparently had no effect on central Pacific deep-water hafnium.
Science↗
Stratigraphy, lithologies, and sedimentary structures of Owens Lake core OL-92
Owens Lake, a now-dry lake in southeastern California immediately east of the southern Sierra Nevada, was the site of a coring project designed to obtain a long paleoclimatic record. During the ensuing study, lacustrine deposits were recovered by the 323 m long core designated “OL-92.” The presence of the Bishop ash (ca. 760 ka) and the Matuyama-Brunhes paleomagnetic reversal (ca. 780 ka) near the base of core OL-92 shows that this core represents about 800 k.y. of deposition in Owens Lake. The sediments are dominantly lacustrine clay, silt, and fine sand, although some intervals contain as much as 40 wt % CaCO3. The lowest ~57 m of recovered sediments is mostly silt or clay, but several sand beds are present; the overlying ~60 m of sediment is similar, but its sand content is more dispersed. Together, these two units are composed of ~70 wt % silt and clay and ~30 wt % sand, suggesting deposition in lakes that fluctuated between moderately deep and shallow. Overlying them is ~201 m of sediments that were mostly deposited in deep water; they consist predominantly of silt and clay but include two thin, coarse-sand beds. An oolite bed forms the upper ~4 m of natural deposits, and an anthropogenic salt bed, >2 m thick, forms much of the present surface. In addition to the Bishop ash, several much thinner tephra layers are also present. About 70% of the clastic-sediment units are massive, some clearly because of bioturbation; other units display a thin bedding defined by changes in color or grain size. Rhythmic bedding, observed in numerous segments <1 m thick, seems to represent cyclical events ~100 yr long. Thin color bands caused by the chemical alteration of sediments on each side of hairline fractures create irregular subvertical veins. Clastic dikes, as much as ~2 cm wide and ~75 cm long, characterize some zones. Bioturbation structures, sand pods, ice-rafted(?) granules, small faults, minor discontinuities, and possible turbidity-current structures are also present. Lithologic variations, in combination with other evidence, indicate that from ca. 810–645 ka, Owens was most commonly a moderately deep fresh-water lake; from ca. 645–450 ka, it was more commonly a shallow—but still fresh-water—lake; from ca. 450–5 ka, it was almost continuously a deep, mostly fresh-water lake; and after ca. 5 ka, it was a shallow, moderately saline lake. Other variations in the sediments and their contents, however, indicate additional cycles of average lake-overflow volumes that are not reflected by sediment-size changes.
California↗
Stratigraphy, correlation, depositional setting, and geophysical characteristics of the Oligocene Snowshoe Mountain Tuff and Creede Formation in two cored boreholes
Core descriptions and geophysical logs from two boreholes (CCM-1 and CCM-2) in the Oligocene Snowshoe Mountain Tuff and Creede Formation, south-central Colorado, are used to interpret sedimentary and volcanic facies associations and their physical properties. The seven facies association include a mixed sequence of intracaldera ash-flow tuffs and breccias, alluvial and lake margin deposits, and tuffaceous lake beds. These deposits represent volcanic units related to caldera collapse and emplacement of the Snowshoe Mountain Tuff, and sediments and pyroclastic material deposited in the newly formed caldera basin, Early sedimentation is interpreted to have been rapid, and to have occurred in volcaniclastic fan environments at CCM-1 and in a variery of volcaniclastic fan, braided stream shallow lacustrine, and mudflat environments at CCM-2. After an initial period of lake-level rise, suspension settling, turbidite, and debris-flow sedimentation occurred in lacustrine slope and basin environments below wave base. Carbonate sedimentation was initially sporadic, but more continuous in the latter part of the recorded lake history (after the H fallout tuff). Sublacustrine-fan deposition occurred at CCM-1 after a pronounced lake-level fall and subsequent rise that preceded the H tuff. Variations in density, neutron, gamma-ray, sonic, and electrical properties of deposits penetrated oin the two holes reflect variations in lithology, porosity, and alteration. Trends in the geophysical properties of the lacustrine strata are linked to downhole changes in authigenic mineralology and a decrease in porosity interpreted to have resulted primarily from diagenesis. Lithological and geophysical characteristics provide a basis for correlation of the cores; however, mineralogical methods of correlation are hampered by the degree of diagenesis and alteration.
Colorado↗
Eastern rim of the Chesapeake Bay impact crater: Morphology, stratigraphy, and structure
This study reexamines seven reprocessed (increased vertical exaggeration) seismic reflection profiles that cross the eastern rim of the Chesapeake Bay impact crater. The eastern rim is expressed as an arcuate ridge that borders the crater in a fashion typical of the "raised" rim documented in many well preserved complex impact craters. The inner boundary of the eastern rim (rim wall) is formed by a series of raterfacing, steep scarps, 15-60 m high. In combination, these rim-wall scarps represent the footwalls of a system of crater-encircling normal faults, which are downthrown toward the crater. Outboard of the rim wall are several additional normal-fault blocks, whose bounding faults trend approximately parallel to the rim wall. The tops of the outboard fault blocks form two distinct, parallel, flat or gently sloping, terraces. The innermost terrace (Terrace 1) can be identified on each profile, but Terrace 2 is only sporadically present. The terraced fault blocks are composed mainly of nonmarine, poorly to moderately consolidated, siliciclastic sediments, belonging to the Lower Cretaceous Potomac Formation. Though the ridge-forming geometry of the eastern rim gives the appearance of a raised compressional feature, no compelling evidence of compressive forces is evident in the profiles studied. The structural mode, instead, is that of extension, with the clear dominance of normal faulting as the extensional mechanism.
Chesapeake Bay↗
Palynology in coal systems analysis-The key to floras, climate, and stratigraphy of coal-forming environments
Palynology can be effectively used in coal systems analysis to understand the nature of ancient coal-forming peat mires. Pollen and spores preserved in coal effectively reveal the floristic composition of mires, which differed substantially through geologic time, and contribute to determination of depositional environment and paleo- climate. Such applications are most effective when integrated with paleobotanical and coal-petrographic data. Examples of previous studies of Miocene, Carboniferous, and Paleogene coal beds illustrate the methods and results. Palynological age determinations and correlations of deposits are also important in coal systems analysis to establish stratigraphic setting. Application to studies of coalbed methane generation shows potential because certain kinds of pollen are associated with gas-prone lithotypes. ??2005 Geological Society of America.
Special Paper of the Geological Society of America↗
Stratigraphy and structure of west-central Vermont
The lithologic units recognizable in the fossiliferous succession along southern Lake Champlain are structurally continuous with and traceable eastward into the “marble belt” of west-central Vermont immediately west of the Green Mountain Front. They are also traceable northward through west-central Vermont into a succession in northwestern Vermont bounded on the east and west by major thrusts, where they pass laterally northeastward into fossiliferous shales, the faunal zones of which are correlated with those along southern Lake Champlain and in the Hudson and Mohawk valleys. The Cambrian strata are traced into west-central from northwestern Vermont whereas the Ordovician correlation is with rocks in the Mohawk-Hudson-Champlain region. Certain of the Upper Cambrian strata can be correlated with established formations of this age in both of the outlying areas. The structural pattern reflects movements dependent upon the original distribution of sedimentary facies. Interbedded Carnbro-Ordovician limestones and dolomites grade westward into foreland sandstones and eastward into geosynclinal shales. Cambrian and early Ordovician sandstone tongues extend far to the east. Later Ordovician strata of the shale facies overlie the calcareous and sandy succession and are locally unconformable on it. In the Taconic Range allochthonous Cambrian strata of shale facies are superposed upon the autochthonous Ordovician beds. The rocks of the klippe were derived from a zone at least 50 miles east of the present westernmost exposures, arriving there by movements confined largely to the shale facies. Flexural folding and thrusting, effects of alternating competency and incompetency of the foreland sequence, affected the latter succession and possibly the Taconic Allochthone. Breaking of competent strata in the flexures initiated the thrusts; thrusting continued by the stripping of competent from incompetent beds. This was accompanied by counterclockwise rotation of the thrust slices around pivotal zones bordering on a foreland massif to the southwest. Thus rock cropping out in the north-south thrust slices originally extended northeast-southwest. The thrusts are concentric to the Adirondack crystallines and are cut by the Adirondack normal faults; probably they were warped during normal faulting and uplift of the Adirondacks.
Vermont↗
Lower Middle Ordovician stratigraphy of the Shenandoah Valley, Virginia
In classifying the lower Middle Ordovician of the Shenandoah Valley, the formation names Stones River, Mosheim, Lenoir, Holston, Whitesburg, and Athens have been used without adequate evidence. Detailed study shows that the so-called Athens and Whitesburg, as developed near Harrisonburg, are laterally continuous with the greater part of the Chambersburg limestone, which is supposed to be younger than the Athens. The newly discovered relations of these formations affect the classification of the Middle Ordovician in much of the northern Appalachian region. The present study has been high-lighted by the discovery that Cryptophragmus antiquatus , widely regarded as a valid guide to the lower Black River, ranges through several hundred feet of beds, possibly as high as lower Trenton. In the Shenandoah Valley, this fossil is most abundant near the top of the Chambersburg, which is supposed to be late Black River or early Trenton. In the proposed reclassification, the lower Middle Ordovician is divided into six time-stratigraphic units, in ascending order: the New Market limestone, Whistle Creek limestone, Lincolnshire limestone, Edinburg formation, Oranda formation, and Collierstown limestone. The Edinburg embraces two equivalent facies: one of cobbly limestone (Lantz Mills facies) which is mainly developed in the northern and western parts of the Shenandoah Valley; and a relatively thicker body of black limestone and shale (Liberty Hall facies) which is typically developed in the Harrison-burg-Staunton area. In the western part of Shenandoah County, the topmost division of the Edinburg formation is composed of light-gray calcilutite and calcarenite, named the St. Luke limestone member. The rusty-brown granular limestones just below Butts' Athens in the Harrisonburg-Staunton-Lexington area are here named the Botetourt limestone member of the Edinburg formation. At least part of the New Market limestone is linked with a part of the New York Chazy and type Lenoir, but the Lincolnshire seems to be post-Chazy. All the succeeding beds, comprising the greater part of the lower Middle Ordovician succession, are Black River or Trenton.
Virginia↗
Stratigraphy of the upper Cambrian, Llano Uplift, Texas
The two formations and eight members that constitute the Upper Cambrian in the Llano uplift of central Texas are described or redefined, and their lithic characters in 19 measured sections are graphically summarized. Standard reference to them is thus furnished. The Riley formation comprises the basal Paleozoic strata of the Llano uplift. Its initial sediments were deposited on a submerged pre-Cambrian terrane having a known topographic relief as great as 800 feet. Its thickness normally averages about 680 feet but ranges from probably less than 200 to about 800 feet. At most places it is subequally divisible between the Hickory sandstone member below and the Cap Mountain limestone member above, with the thin but widespread, glauconitic Lion Mountain sandstone member capping and completing the sequence. The Wilberns formation includes five named members between the Riley formation and rocks of the Lower Ordovician Ellenburger group. It normally averages about 580 feet thick and ranges from 540 to 610 feet thick, but in the southeastern corner of the Llano uplift truncation of the upper beds has reduced it to 360 feet. The thin but widespread, nonglauconitic Welge sandstone member introduces the sequence. Above it is the Morgan Creek limestone member, grading to the succeeding argillaceous beds of the Point Peak shale. At the top of the sequence are the San Saba limestone and Pedernales dolomite members. These two are essentially equivalent and gradational facies, with the Pedernales normally overlying the San Saba.
Texas↗
Structure of the Sugarloaf Mountain area, Maryland, as a key to piedmont stratigraphy
No abstract available.
Maryland↗
Stratigraphy of Upper Pennsylvanian and lower Permian rocks in the Sand Canyon area, Otero County, New Mexico
The Magdalena Group of Pennsylvanian age and, in ascending order, the lower tongue of the Abo Sandstone (herein named the Danley Ranch Tongue), the Hueco Limestone, the upper tongue of the Abo Sandstone (herein named the Lee Ranch Tongue), and the Yeso Formation of Permian age are well displayed in the southern part of the Sacramento Mountains. The contact of the Pennsylvanian rocks with the overlying Permian rocks is unconformable throughout the area. Locally the relief before deposition of the Abo was so great that in places the Danley Ranch Tongue of the Abo is absent because of nondeposition. At these places there is prominent angular discordance between the Pennsylvanian and Permian rocks. The red beds of the Danley Ranch Tongue interfinger with and grade southward into the overlying Hueco Limestone. Red beds of the Lee Ranch Tongue of the Abo Sandstone also grade southward into the Hueco and cannot be traced south of the southern part of the Sacramento Mountains. The Lee Ranch Tongue is believed to be stratigraphically lower than the Deer Mountain Red Shale Member of the type Hueco Limestone. The herein named Otero Mesa Member of the Yeso Formation is believed to occupy an intermediate stratigraphic position between the Lee Ranch Tongue and the Deer Mountain Red Shale.
New Mexico↗
Stratigraphy of ocoee series, Great Smoky Mountains, Tennessee and North Carolina
Much of the Great Smoky Mountains, which span the boundary between Tennessee and North Carolina, is formed of the Ocoee series, of later Precambrian age. This is a body of terrigenous clastic sedimentary rocks, which has minor intercalations of limestone and dolomite but no volcanic components or known fossils. The series is probably 30,000 feet or more thick. It lies unconformably on a basement of earlier Precambrian granitic and gneissic rocks, and on the northwest side of the mountains it is overlain by the Cochran formation, or basal unit of the Chilhowee group, which is of Cambrian and Precambrian(?) age. South of the mountains it is overlain by rocks of the Murphy marble belt; here, the top of the Ocoee is placed tentatively at the base of the Nantahala slate. The Ocoee series is divisible into three broad units of regional extent and contrasting lithologic character, which are herewith designated groups and named the Snowbird group, the Great Smoky group, and the Walden Creek group. The groups consist of local intergrading and intertonguing formations and have complex stratigraphic and structural relations. The Ocoee series is split by major thrust faults into three sequences, a southern, central, and northern, none of which contains more than two groups of the series. The lowest group, the Snowbird, is best developed in the central sequence where it is more than 13,000 feet thick; it is much thinner in the southern sequence. In both sequences it lies unconformably on granites and gneisses. Toward the east it is largely sandstone, but this is interbedded with and intertongues westward into finer-grained sandstone, siltstone, and argillaceous rocks. These differences permit the Snowbird group to be divided, in ascending order, into the Wading Branch formation, Longarm quartzite, Roaring Fork sandstone, and Pigeon siltstone. Toward the west the Snowbird is represented by more strongly metamorphosed rocks termed the Metcalf phyllite. Above the Snowbird group in the southern sequence is the Great Smoky group, more than 25,000 feet thick. The lower part, termed the Elkmont sandstone, is fine-grained sandstone. The middle part, termed the Thunderhead sandstone, is poorly sorted, coarse, feldspathic sandstone in graded beds. The upper part, termed the Anakeesta formation, includes many beds of dark argillaceous and silty rocks. South of the Great Smoky Mountains the Great Smoky group is overlain by the rocks of the Murphy marble belt. The Snowbird group in the central sequence is overlain in a few places by coarser-grained rocks. These somewhat resemble those of the Great Smoky group, but their stratigraphic relations to it are not demonstrable, so that they are left as unclassified parts of the Ocoee series. In one area such rocks are named the Rich Butt sandstone, in another the Cades sandstone. In the northern sequence the Walden Creek group, about 8000 feet thick, forms a varied assemblage of argillaceous and silty rocks and quartz-pebble conglomerate, with locally prominent quartzite, limestone, and dolomite. The Walden Creek group is divided, in ascending order, into the Licklog formation, Shields formation, Wilhite formation, and Sandsuck formation; the Wilhite is in turn divided into the Dixon Mountain member and Yellow Breeches member. The Walden Creek is overlain by the Chilhowee group of Cambrian and Precambrian(?) age but is in fault contact with the Snowbird group on the southeast; it is not in contact with the Great Smoky group. Northeast of the Great Smoky Mountains, the Walden Creek group is reported to overlie the Snowbird group as part of a sedimentary sequence from the basement rocks upward through the Chilhowee group into the overlying Paleozoic. © 1958, The Geological Society of America, Inc.
Tennessee, North Carolina↗
Stratigraphy and structure at the north end of the taconic range in west-central Vermont
No abstract available.
Vermont↗
Stratigraphy and heavy minerals of the bays formation, Bays Mountain synclinorium, northeast Tennessee
The Bays Mountain synclinorium is in the Valley and Ridge province in northeast Tennessee , southwest of Kingsport and west of Greeneville. The more clastic part of the Bays formation lies in the east section of the synclinorium . The thickness of the Bays decreases from about 870 feet on the east to about 600 feet on the west. Presumably, the red beds and primary features of the Bays formation formed under deltaic conditions. A Camarocladia Zone near its base indicates a time-transgression of the Bays . The Bays is older at the base to the east than at its base to the west. The heavy minerals of the Bays were compared with those of the Cambrian and Precambrian(?) sediments, volcanic rocks, and accessory minerals of the crystalline complex which now comprise the Blue Ridge. The minerals identified include nearly all those in the Cambrian and Precambrian(?) sediments as well as additional minerals not found in these older sediments but that are in the crystalline complex. Such minerals indicate that at least a part of the crystalline complex was above sea level during the Middle Ordovician.
Tennessee↗
Pre-Olympia Pleistocene stratigraphy and chronology in the central Puget Lowland, Washington
Drifts of two pre-Olympia glaciations separated by nonglacial sediments are widespread in the central Puget Lowland of western Washington. The Double Bluff Drift (older) and Possession Drift represent advances of the Puget lobe of the Cordilleran ice sheet more than 40,000 years ago. The nonglacial Whidbey Formation between the drifts was formed in streams and lakes. During its deposition, climate was initially cool and moist, as inferred from pollen in peat beds, but subsequently it became much like that of the present in the lowland. The Possession Drift is tentatively correlated with glacial deposits of Salmon Springs age in the southern part of the lowland. The Whidbey Formation may correlate with nonglacial deposits between two Salmon Springs Drifts or with the Puyallup Formation.
Washington↗
Stratigraphy and correlation of the precambrian belt supergroup of the southern Lewis and Clark Range, Montana
Several well-exposed and little-deformed Belt Supergroup sections have been studied in the southern Lewis and Clark Range . In the area studied, the Belt thins eastward or northeastward due both to primary sedimentation and to pre-Middle Cambrian erosion. These rocks can now be more precisely correlated with the well-known sections near Bonner, Helena, and Glacier National Park. In the western part of the area, the Missoula Group is thickest and lithologically intermediate between the sections at Bonner and Marias Pass. Formation names from these two sections are applied in the southern Lewis and Clark Range . The thin Belt sequence in the eastern part of the area includes the lower part of the Missoula Group and older formations that may be traced southward into the Helena, Empire, and Spokane Formations of the Helena area. Consequently, the Helena Dolomite of the Helena area, the Siyeh Formation of the Marias Pass area, and the "Newland Limestone" of the Bonner area are probably lateral equivalents.
Montana↗