Geology topics
Publications and source records attributed to William P. Elder.
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Upper Cretaceous limestone-shale couplets developed within the late transgressive stage of the Greenhorn cyclothem may be correlated from carbonate-dominated (basinal) sequences in central Kansas and Colorado westward to clastic cycles in southern Utah. Six such basinal couplets have been traced to corresponding upward-coarsening progradational cycles developed on the western margin of the Western Interior basin. In the central basin in Colorado and Kansas, these sedimentary cycles are represented by limestone-shale and marlstone-shale couplets ∼0.5-1.0 m in thickness. More calcareous parts of these couplets may be correlated westward into condensed, fossiliferous concretion and shell beds in proximal offshore lithofacies of Arizona and Utah. These concretion and shell beds are physically traceable farther landward (westward) into bioturbated, fossil-rich, transgressive lag deposits that bound 10- to 20-m-thick coarsening-upward progradational strand-plain deposits (parasequences) in southwestern Utah. Thus, the progra-dational phase of parasequence deposition correlates with accumulation of clay-rich sediment in the central basin, and the transgressive phase is characterized by reduced terrigenous input and deposition of carbonate-rich sediment. We consider Milankovitch-style orbital forcing of climate and tectonically induced fluctuations in rates of foredeep basin subsidence as possible forcing mechanisms for these basinwide events. Based on the widespread distribution of the limestone-shale couplets, as well as on estimated sedimentation rates and geochronology, it has been widely speculated that these carbonate cycles reflect Milankovitch cycles with periodicities on the order of 20 k.y. to 100 k.y. If so, then stratigraphic data suggest that orbital forcing of climate affected eustasy and/or sediment input and biogenic production in the Western Interior basin. Alternatively, thrusting events in the Sevier orogenic belt may have produced episodic changes in the rates of foredeep basin subsidence and consequent changes in base level, which could have controlled the deposition of the Greenhorn parasequences and carbonate cycles. In either case, correlation of these units demonstrates a consistent basinwide sedimentary response to high-frequency base level or sediment input changes in the Western Interior epicontinental basin.
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The Pigeon Point Formation crops out along the San Mateo County coastline in a northern and southern sequence of folded and faulted strata. Correlation of the two sequences remains somewhat equivocal, although on the basis of biostratigraphy and a reversed magnetic interval both appear to have been deposited during the early to middle Campanian. Sedimentary structures suggest that the northern sequence was deposited by turbidity currents in a continental rise setting, whereas the southern sequence primarily reflects deposition in shelf and slope environments . Right-lateral offset on the San Andreas and subsidiary faults to the east of the Pigeon Point Formation can account for 100's of km of northward transport since its deposition. However, Champion and others (1984) suggested 2500 km of northward transport from a tropical setting of about 21°N. Molluscan assemblages in the formation argue strongly for a less tropical site of deposition. Relative abundances of warm and temperate taxa and the presence or absence of key species are similar to those of the Santa Ana Mountains Cretaceous section, and are indicative of a war
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Lithology, carbonate and organic carbon geochemistry, and molluscan macrofauna were investigated in fine-grained clay-dominated strata spanning the Cenomanian-Turonian Stage boundary in the southern Colorado Plateau region. Analyses of these data show basinal and onshore to offshore changes in sedimentologic and molluscan assemblage trends through the upper Cenomanian Sciponoceras gracile and Neocardioceras juddii and lower Turonian Watinoceras coloradoense Biozones. Diverse infaunal/epifaunal bivalve assemblages are best developed in the Sciponoceras gracile Biozone and at the more clay-rich nearshore sections. Gastropod and infaunal bivalve abundance and diversity decrease in an offshore and up-section direction as carbonate and organic carbon percentages generally increase. The Neocardioceras juddii Biozone precedes the stage boundary and is characterized in Utah and Arizona by pulsed increases in organic carbon and detrital-feeding gastropods, and by a shift in infaunal bivalve dominance toward species adapted for soft-substrate conditions. These data suggest that pulses of increased organic detrital resources and decreased substrate firmness were associated with the stage boundary extinctions. The stage boundary is marked by a change to depauperate epifaunal assemblages at western sections. At all sections, the base of the Watinoceras coloradoense Biozone is reflected by an abrupt increase in carbonate and decrease in organic carbon percentages as well as by the immigration of Mytiloides bivalves. Periods of progressive recovery through this biozone were associated with carbonate-rich depositional periods.
No abstract available.
This field trip covers part of the Diablo Range and adjacent San Joaquin Valley of central California (Fig. 1 ). The core of the range is made up of rocks of the Franciscan Complex, flanked by Coast Range ophiolite (CRO) and Great Valley sequence (GVS). The Franciscan Complex in this area consists of deformed and metamorphosed sedimentary and volcanic rocks containing fossils of Late Jurassic to Late Cretaceous age. These rocks are believed to represent an accretionary wedge that was subducted to depths of 12-20 mi (20-30 km). The Middle to Late Jurassic CRO represents a slab of oceanic upper mantle and crust that was trapped between the Sierran magmatic arc and the Franciscan trench. The Upper Jurassic to Upper Cretaceous GVS is a thick accumulation of mudstone, sandstone, and con- glomerate that was deposited on the ophiolite in a forearc-basin setting. The objectives of this field trip are to examine good exposures of these three major units in order to better understand their sedimentary, igneous, and metamorphic histories, to examine some of the major faults bounding the units, and to gain an understanding of the tectonic history of this portion of the Coast Ranges.
No abstract available.
If at least some mass extinctions are caused by impacts, why do they extend over intervals of one to three million years and have a partly stepwise character? The solution may be provided by multiple cometary impacts. Astronomical, geological and palaeontological evidence is consistent with a causal connection between comet showers, clusters of impact events and stepwise mass extinctions, but it is too early to tell how pervasive this relationship may be.
The Golden Trout Wilderness covers 457 mi 2 in the rugged forested part of the southern Sierra Nevada. It is bounded on the east by Owens Valley, on the north by Sequoia National Park, and on the west and south by U.S. Forest Service multiple-use lands. The east side of the area is accesible from U.S. Highway 395, while the remainder is reached by state, county, and Forest Service roads from the San Joaquin Valley.