Browsing by Subject "Biostratigraphy"

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Effects of taxonomic and locality inaccuracies on biostratigraphy and biochronology of the Hueso and Tapiado formations in the Vallecito Creek-Fish Creek section, Anza-Borrego Desert, California
(2008-12) Murray, Lyndon Keith; Bell, Christopher J., 1966-
The fossiliferous sediments of the Hueso and Tapiado formations exposed in the Vallecito Creek-Fish Creek section (VCFC) of the Anza-Borrego Desert (ABD) span the boundary between Blancan and Irvingtonian North American Land Mammal 'Ages.' Historically, the determination of the Blancan-Irvingtonian (B-I) boundary in the VCFC proved problematic. A new study to determine the B-I boundary in the VCFC revealed data inaccuracies in both published works and curatorial records of the mammal and other vertebrate fossils from the ABD. Most individual inaccuracies were minor, but an accumulation of over 50 years of multiple inaccuracies had significant effects on local and regional biostratigraphic and biochronologic correlations. A detailed investigation of the inaccurate data resulted in recognition of 17 types of error, with at least five sources. The two most prominent sources of error are those derived from publication and curation protocols. Examination of over 150 publications and in-house faunal lists produced over 830 taxonomic names and format variants, for 110 mammalian taxa identified from the ABD and VCFC. Approximately 50% of the taxonomic identifications were previously published without voucher catalogue numbers or fossil descriptions. A critical review of the taxonomic assignments resulted in an updated faunal list of ABD terrestrial Mammalia, including 110 taxonomic names, 66 unqualified genera, and 46 unqualified species. A supplemental list of 'retired' taxa includes 178 previously published or listed taxonomic names and format variants. The 4.5 km sequence of originally superposed sediments within VCFC is now tilted at 23 degrees and exposed in plan view. This exposure was captured in both aerial photos and satellite imagery. GIS layers of the Geomagnetic Polarity Time Scale and individual collecting localities are readily superposed onto the stratigraphic images. When united with the faunal database, GIS maps of biostratigraphic data facilitate detection and correction of data errors. Resulting corrected maps show highest and lowest stratigraphic occurrences of taxa, as well as geographic clustering of taxonomic groups, outlining possible paleohabitats. As a result of data improvement and GIS display, the local B-I boundary currently is best estimated by the presence of Ovibovini at >1.42 Ma.
Sand distribution along shelf-edge deltaic systems : a case study from eastern offshore Trinidad
(2010-12) Davila-Chacon, Anmar Carolina; Moscardelli, Lorena Gina, 1977-; Wood, Lesli J.; Fisher, William; Steel, Ronald
The study area is situated along the obliquely converging boundary of the Caribbean and South American plates offshore eastern offshore Trinidad. Major structural elements in the shelf break and deep-water slope regions include normal and counter-normal faults to the south and large transpressional fault zones to the north. Well logs and biostratigraphic information were analyzed for twenty-four wells in the study area to refine previous depositional environment interpretations. For purposes of this net sand distribution analysis it was decided to consider the deltaic portion of the shelf transit cycle, against the marine portion of the shelf transit cycle and were named T and R cycles, respectively. T and R cycles were interpreted based on well log patterns and depositional facies shifts. Six T/R cycles were interpreted within the Pliocene to recent stratigraphic succession and shelf edge trajectories were also mapped for each of these cycles based on earlier stratigraphic correlations. Net-to-gross (NTG) ratios were calculated for each component of the T/R cycles and plotted against total thicknesses and net sand values. In addition, NTG trends were mapped for each interval and analyzed based on their proximity to the corresponding shelf edge. Mapping of the shelf edge trajectories (SET) revealed that (1) SET migrate northeasterly across the Columbus Basin through time and (2) shelf edge orientations are parallel to the strike of growth faults in the south but deflect to the northeast near the Darien Ridge indicating a strong underlying structural control. The NTG plots and maps also revealed that (1) For T cycles, NTG values never exceed 60% and are inversely proportional to total thickness, (2) For R cycles, NTG values are highly variably ranging from 35% to 90%, (3) NTG values increase as the shelf break is approached and (4) The distribution of NTG ratios is also controlled by accommodation space created by local structures. The Guiana current is believed to play an important role in the redistribution and reworking of sand in the Columbus Basin. Aggradation and progradation distances were computed for each interval and the results suggest that the younger Sequences C2 (T-R cycle E) and C3 (T-R cycle F) show a stronger progradational trend than the older C4, C5 and C6. This strong progradational trend might indicate delivery of sand basinwards, while for the older intervals; the aggradational trend suggests an increase in sediment storage. In long-term scale (1-2 m.y.) the Orinoco Delta seems to behave as an aggradational delta that increases sediment storage due to growth fault and high subsidence rates. However, in the short-term scale, the Orinoco delta seems to behave as a rapid progradational delta, for the younger sequences C2 and C3, where sediment bypass is more likely to occur; and as a rapid aggradational (slow prograding) margin for the older intervals C4, C5 and C6.
The combined effects of river-flood and tidal processes on the stratigraphy, ichnology, and stratal architecture of the Cretaceous (Campanian) Loyd Delta near Rangely, Colorado U.S.A.
(2016-05) Prather, Timothy John; Fisher, W. L. (William Lawrence), 1932-; Flaig, Peter Paul; Mohrig, David
The Campanian Loyd Sandstone (Loyd) has been identified in outcrops in the Piceance Basin within strata above the Castlegate Sandstone near Rangely, CO. The Loyd preserves sedimentary structures, facies, sandbody/shale geometries, and stratal architectures characteristic of a mixed river-flood dominated tidally influenced delta. Characteristics include abundant low angle planar laminated foresets, flaser-wavy-lenticular bedding between foresets, trough cross-stratified mouth bars and subaqueous terminal distributary channels, and hyperpycnites on the distal delta front to prodelta. The Loyd contains a surprisingly high abundance, high-diversity fully marine ichnologic assemblage for a delta with river-dominated characteristics. The bulk of these trace fossils are found within and at the base of low-angle planar laminated sandstone beds, and in flaser-wavy-lenticular beds. Traces display a tiered relationship, with some burrows penetrating multiple beds on a meter scale. Despite strong evidence in sedimentary structures and sandbody geometries of river-dominance, the high abundance high diversity trace fossil assemblage and tidal indicators suggest that the bulk of time on the Loyd delta-front is recorded by tidally modified fine-grained sediment deposition and extensive bioturbation. Extended periods of low-energies, low sedimentation rates, and relative quiescence were likely punctuated by deposition of sandy traction flows and channelization during river-flood conditions. Although the Loyd is grossly similar to the river-flood dominated Panther Tongue and Ferron deltas, and contains structures similar to tide-dominated deltas of the Sego sandstone, it should not be classified as an endmember example of either type of delta. Integrating ichnological analysis to determine how processes fluctuate during an episode of deltaic deposition allows for a refinement of the common delta classification scheme.