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Where: Zoom
Cost: $10
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In-Person
Where: The Petroleum Club of Shreveport, 15th floor
Cost: $20, Children 10 and under $8
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Biography
Stacy Atchley received both his B.S. and M.S. degrees from Baylor University during the 1980s and Ph.D. from the University of Nebraska, Lincoln in 1990. Upon graduation he worked for seven years for Exxon Company, U.S.A., Exxon Exploration, and Exxon Production Research. Since his time at Exxon, Dr. Atchley has served (and currently serves) as director of the Applied Petroleum Studies (APS) program at Baylor University. Within the APS program, Dr. Atchley has directed applied student research projects in the Anadarko, Permian and Alberta Basins related to both production optimization and regional exploration assessment.
Abstract
Modern sequence-stratigraphic theory has its foundation in the works of L.L. Sloss and W.C. Krumbein (1940s to 1960s) and several Exxon researchers (1970s to 1990s). These works largely focus on the nature and origin of sedimentary cycles within marine stratal successions. Since that time, sequence-stratigraphic concepts have evolved to include the analysis of terrestrial strata. Historically, the recognition of unconformity-bounded cyclic stratal units (such as sequences) has relied upon the geometric relationships of strata, i.e., onlap, toplap, truncation and downlap, within two- and/or three-dimensional outcrop or subsurface successions. Oftentimes, however, outcrops or boreholes are isolated, and do not preserve these diagnostic stratal relationships. In such instances, documentation of changes in the vertical succession of strata, rather than lateral, may allow reconstruction of the cyclic accommodation history and placement of associated bounding discontinuities. This technique, referred to as "stacking pattern" analysis, was originally developed for shallow-marine carbonate successions. More recently, the stacking pattern methodology has been similarly applied to alluvial successions, and takes into account the unique processes of terrestrial deposition and pedogenesis. The most conspicuous, and fundamental cyclic stratal unit recognized within alluvial settings are fluvial aggradational cycles (FACs). FACs are meter-scale, typically fining-upwards successions that have a disconformable lower boundary and an upper boundary that either has a paleosol weathered into it, or is disconformably overlain by the succeeding FAC without a paleosol. FACs are thought to represent sediment accumulations during channel avulsion events that are subsequently weathered during the following period of channel stability. A thick succession of FACs indicates sediment accumulation during a prolonged episode of accommodation gain. Variations in the rate of accommodation gain (and loss) are interpreted to result in the organization of FACs into alluvial sequences, and longer period composite sequences. Episodes of base-level rise result in relatively rapid rates of alluvial aggradation and less developed and more poorly-drained paleosols. Associated FACs are thicker than average and transition from initially lower sinuosity-, higher-competence alluvial systems to comparably higher sinuosity, lower competence channel deposits. As base-level rise decelerates and initially falls, paleosols become increasingly well-developed and better drained, and FACs are thinner than average and transition to even lower competence, higher sinuosity channel sandstones that are more extensive due to prolonged channel migration under low accommodation conditions. During base-level fall, the incisement of alluvial valleys produces sequence boundaries that are infrequently flooded across interfluve areas. FACs across interfluve positions are much thinner than average and are characterized by the most well-developed and best-drained paleosols.
Application of the alluvial stacking pattern methodology is demonstrated within three case studies. Case study 1 from Big Bend National Park, Texas, considers a latest Cretaceous to earliest Tertiary passive margin, coastal plain succession, and correlates alluvial sequences and associated climate and ecosystem changes to eustatic sea level oscillations. Case study 2 from northern and northeastern New Mexico, documents a Late Triassic foreland basin succession where tectonically-induced accommodation events are correlated between isolated outcrop successions that are 200 km apart. Case study 3 from central New York, demonstrates how stacking pattern analysis allows correlation of a Middle Devonian alluvial composite sequence with equivalent regressive-transgressive marine strata along a convergent plate boundary.
