Edward L. Winterer

Subsidence and Sedimentation on Jurassic Passive Continental Margin, Southern Alps, Italy

The southern Alps of Italy preserve a tectonically intact array of Jurassic facies that record the evolution of a part of the margin of the Apulian plate from its ancestral beginnings in a complex of Permian and Triassic rifted continental basins through the initial stages of breakup and stepwise foundering of a carbonate platform. Breakup was accompanied first by the rapid accumulation of thick prisms of carbonate turbidites in newly formed fault troughs. Then, as the new Ligurian oceanic basin began to open farther west and, as subsidence gradually slowed, accumulation of a succession of slowly deposited biogenous pelagic sediments recorded not only the increasing depths of the seafloor but also fluctuations in oceanographic conditions of fertility, carbonate dissolutio levels, and the strength of bottom currents. Estimates of the history of seafloor depths, based on a simple subsidence law of the form Subsidence = K(Age)12/, provide a basis for the construction of a set of curves showing the changing depths of significant carbonate dissolution surfaces during the Jurassic in this region. The rapid 1-km deepening of the compensation depth for calcite during the Late Jurassic may be due to a change in regional oceanic vertical circulation patterns from upwelling (fertile, silica-rich, carbonate dissolving) to downwelling (less fertile, silica-poor, carbonate preserving).

Sedimentary Facies and Plate Tectonics of Equatorial Pacific

Facies patterns of post-Eocene equatorial Pacific pelagic sediments indicate northward motion of the Pacific plate of about 3 cm/year with respect to the earths spin axis, whereas the age gradient of volcanoes in the Hawaiian chain indicates a northward component of motion of the plate with respect to a hot spot in the subjacent asthenosphere of about 6 cm/year. The discrepancy is ascribed to southward motion of the upper part of the asthenosphere with respect to the spin axis. To examine this possibility more closely, a theoretical isopach map is generated for post-Eocene equatorial sediments, to compare with thicknesses known from drilling and seismic studies. In the model, sediment accumulation rates at all depths are several times faster close to the equator than farther away. The sedimentation model is combined with a plate-motion model with the following elements: (a) Pacific plate moves westward away from the rise crest at 10 cm/year; (b) sea floor deepens gradually as it moves away from the rise, with constant crustal-age versus depth relation; (c) plate moves northward with respect to the earths spin axis at 3 cm/year; and (d) hot spots in the asthenosphere, which give rise to chains of volcanoes, move southward with respect to the spin axis, with ame speed. The model isopachs correspond fairly well to the known sediment thicknesses. Details of pre-Oligocene motions of the plate versus the earths spin axis are uncertain, but mid-Cretaceous volcanoes have been shifted about 30° north. Northwest motion of the plate over hot spots in the asthenosphere at about 5 cm/year generated seamount chains parallel with the Line Islands in the period between about 30 and 100 m.y. ago.

Silurian Reef Complex and Associated Facies, Central Nevada

The Silurian System of the Great Basin comprises at least three major facies: (1) a dolomite fades, developed in Utah and in eastern and central Nevada (Laketown dolomite and Lone Mountain dolomite), (2) a limestone facies, locally dolomitized, exposed in central Nevada (Roberts Mountains formation), and (3) a chert-shale facies, probably deposited originally in central and western Nevada, but since carried eastward into central Nevada by large-scale thrust faulting. Mapping in the Roberts Mountains region in central Nevada shows that the Lone Mountain dolomite and the Roberts Mountains formation are largely, and perhaps wholly, lateral equivalents. Evidence from textures, structures, and fossils indicates that the Lone Mountain dolomite represents a reef complex, most of whose original features are obliterated by dolomitization, and that the Roberts Mountains formation comprises deeper-water reef-flank, off-reef, and basin deposits. A regional isopach map shows a generally north-trending band through the Roberts Mountains in which the Silurian is as much as four times as thick as the regional average for the system-that is, 4,500 versus 1,000 feet. The reef complex of the Lone Mountain dolomite merges with the thinner but lithologically similar Laketown dolomite to the east but interfingers abruptly with the lithologically dissimilar Roberts Mountains formation, which thins very rapidly westward.

Devonian Stratigraphy of Sulphur Springs and Pinyon Ranges, Nevada

Four widespread and distinctive lithologic subdivisions of the autochthonous Devonian strata, the Nevada limestone of Hague, are recognized in an area 25-70 miles north of Eureka, Nevada. The three lower units, (1) the McColley Canyon member of limestone and dolomitic limestone, (2) the Union Mountain member of quartzite, quartzitic dolomite, and dolomite, and (3) the Telegraph Canyon member of light- and dark-banded mottled dolomite with a minor limestone tongue, are placed within the Nevada formation as redefined by Nolan, Merriam, and Williams. The uppermost unit, consisting of limestone truncated above by the Roberts Mountains overthrust, is assigned to the Devils Gate limestone, similarly redefined. The Nevada formation contains faunas ranging from those of the Trematospira (Oriskany) zone to Stringocephalus and Stromatoporoid-Cladopora (Middle Devonian). The Devils Gate limestone faunas range in age from Middle Devonian to early Late Devonian. The Nevada formation is regarded as a lithic correlative of the Simonson and the major part, if not all, of the Sevy dolomites of western Utah and possibly of parts of the Hidden Valley and Lost Burro formations of the Panamint Range, California. The Devils Gate limestone is considered an equivalent of the Guilmette formation of western Utah.

Historical Geology of the Pacific: ABSTRACT

The development over the past few years of a high-resolution biostratigraphic scheme based on joint occurrences of Radiolaria, nannofossils, and planktonic Foraminifera enables us to make new progress on problems of paleogeography, paleoceanography, sedimentation, and tectonics in the Pacific. Results from the Deep Sea Drilling Project, combined with other geologic and geophysical data, suggest the following post-Jurassic history for the Pacific plate. Sedimentary facies patterns reflect a northward motion of the plate relative to the equatorial zone of high biologic fertility, as well as progressively increasing sea-floor depths as newly formed crust moves away from the East Pacific Rise. Superimposed on these gross patterns are evidences of fluctuations in the width of the zone of high productivity, changes in the calcium carbonate compensation depth, and variations in the intensity of bottom-water circulation. Extensive Early Cretaceous volcanism inundated much of the older western part of the plate and was succeeded by the building and subsidence of long chains of seamounts as the plate moved northwestward, possibly over hot spots beneath the lithosphere. Comparison of plate motion, as indicated by equatorial-zone sediments, with motion indicated by trends and age progressions in seamount chains leads to the hypothesis of a south-moving counter flow in the asthenosphere. End_of_Article - Last_Page 440------------

Tectonic Erosion in the Roberts Mountains, Nevada

In central Nevada the mid-Paleozoic Roberts thrust telescopes very unlike facies of lower Paleozoic rocks, originally deposited many scores of kilometers apart. During early Cretaceous times this tectonic sequence was intricately reshuffled by gravity sliding. The slides moved as much as 15 km. radially outward from the crest of a structural high, probably raised by a still-hidden intrusive body, and carried away thick sections of strata from both the upper and lower plates of the Roberts thrust, jumbling and interleaving them in the process. Removal of the effects of these younger movements and complications permits the restoration of a simple geometry on the original Roberts thrust.

Biogenous Sediments Recovered by Deep-Sea Drilling: ABSTRACT

The Deep Sea Drilling Project has provided about 60,000 m of cores that contain a record of biogenous sedimentation over a major part of the world ocean during the past 150 m.y. Subduction and subsidence bias the record in older strata toward sediments deposited near rise crests, and technical drilling problems bias the samples toward low latitudes. After factoring out the effects of plate motions and subsidence, the main features of maps of post-Jurassic biogenous facies reflect primarily the patterns of oceanic fertility and of dissolution of carbonates with depth. These in turn respond to changes in the interacting climate and the deep and surface oceanic circulation systems, which are ultimately determined by the changes in locations, shapes, and interconnections of t e ocean basins and their marginal seas. One great value of the cores is in their being samples whose biostratigraphic age is precisely known, whose paleolatitude, paleolongitude, and paleodepth can be specified, and whose pressure-temperature and pore-water history during burial and diagenesis generally can be far better constrained than for most sediments on land. Biostratigraphers and paleoenvironmentalists End_Page 553------------------------------ have been more active in exploiting these properties than have sedimentary petrologists interested in understanding the processes of diagenesis and lithification of calcareous and siliceous sediments. Lithologic criteria indicate very small volumes of oceanic biogenous sediments of post-Jurassic age are exposed on land, and it is questionable if any but relatively tiny amounts of any age have ever been added to the continents. End_of_Article - Last_Page 554------------

A Formation of Late Miocene and Early Pliocene Age on the North Slope of the Santa Susana Mountains, California: ABSTRACT

It is proposed to recognize a new formation for coarse- and fine-grained sedimentary rocks of late Miocene and early Pliocene age on the north slope of the Santa Susana Mountains in the east-central part of the Ventura basin. These rocks were included by Kew (1924) in the Modelo formation. They overlie and intertongue with shale of late Miocene age in the Modelo formation, and are overlain by the Pico formation of Pliocene age. The formation is well exposed near Tapo Canyon in the type region straddling the boundary between Los Angeles and Ventura counties. Two main types of sedimentary rocks are represented in the new formation: generally light-colored sandstones and conglomerates, and brown-weathering shales and mudstones. Marked lateral changes in lithology are common. The coarser-grained rocks are lenticular, but the shales and mudstones are fairly persistent. In the type region both the lower and upper boundaries are apparently gradational. In any one section the first thick-bedded pebbly sandstone lens above the shales and thin-bedded sandstones of the Modelo marks the base of the new formation. The top is drawn at the base of the first soft olive-gray siltstone containing rusty concretions, a type of lithology characteristic of the Pico formation. Along the north limb of the Pico anticline the formation has the following exposed thickness: East Canyon, 1,825 feet; Wiley Canyon, 1,750 feet; Towsley Canyon, 2,200 feet; Pico Canyon, 3,000 feet. At Tapo Canyon the formation is 4,350 feet thick. Vertical grading is present on two scales. Units tens of feet thick consist of conglomerate at the base, changing upward to thick-bedded sandstone, then alternating sandstone and mudstone, and finally mudstone at the top. Individual beds within these large units commonly are graded. This grading, the abundance of extremely angular blocks of mudstone as clasts in the conglomerates, and the presence of lump structures throughout the formation suggest the possibility that submarine slumps and turbid currents were important agents in the deposition of the coarser-grained rocks. End_of_Article - Last_Page 2631------------

East Ventura Basin Cross Section: ABSTRACT

Beginning at the granite-Eocene contact in the northeast part of the Ventura sedimentary basin, the cross section proceeds southeasterly through the Oak Canyon, Del Valle, Newhall-Potrero and Pico Canyon oil fields. From Pico Canyon the section is projected along a surface rock datum to Rice Canyon, then due south through the Aliso Canyon oil field. Rocks of Pleistocene through Eocene ages are discussed. Purpose of the cross section is to illustrate the relationships between various time-stratigraphic and rock-stratigraphic units. Comments are made on the validity and accuracy of surface and subsurface formation names. Need for a new formation name for rocks lying between base of Pico formation and top of the Modelo formation is discussed. End_of_Article - Last_Page 2632------------