46096-G8
Refining Sequence Stratigraphic Interpretations Using Benthic Fossil Communities from the Permian Basin of West Texas
Sequence stratigraphy focuses on understanding and predicting rock types and stratigraphic surfaces in sedimentary successions and is an important tool for exploration and development of hydrocarbon resources. Within the context of sequence stratigraphy, fossils provide critical information about depositional conditions. However, the very factors that generate the sequence architecture of the stratigraphic record (i.e., changes in sea level, climate, sediment supply, and tectonism) represent environmental perturbations that can profoundly influence the structure of ecological communities. Understanding the response of species associations to environmental change is essential to successfully using fossils to refine and test sequence stratigraphic interpretations. A central long-term goal of my research program is to enhance the use of fossil community analysis in sequence stratigraphic interpretation. The specific objective of this study is to identify the effect of environmental change on the species abundances and associations of fossil communities using brachiopod data from the Permian Basin.
Research Objective #1: Establish an environmental context for observed changes in brachiopod associations of the Bell Canyon Formation. Excellent exposures of the Bell Canyon Formation (Capitanian stage; middle Permian) in the Guadalupe Mountains record a succession of reef slope and toe-of-slope limestones intertongued with basinal sandstones. When this study started, the uppermost two carbonate units of the Bell Canyon (the Lamar and Reef Trail Members) were known to have a distinctly different brachiopod fauna from the lower units of the formation, but the reasons for this difference were not known. It was not clear whether this ecological change reflected facies shifts along the reef margin or fundamental reorganization of brachiopod communities in response to a basin-wide environmental perturbation recorded as a sequence boundary at the base of the Lamar. Distinguishing these two possible explanations has required tying specific brachiopod associations to lithofacies based on outcrop and microscopic description. More than twelve weeks of fieldwork over two summers has resulted in a set of detailed measured sections and over 50 new fossil collections through the Bell Canyon, including areas that had not previously been studied in detail. Initial results confirm that brachiopod associations in the Lamar and Reef Trail are distinct in terms of species composition, abundance, and associations compared to the lower Bell Canyon despite coming from the same lithofacies. This suggests that the brachiopod communities experienced reorganization in response to environmental perturbation rather than tracking gradually changing depositional conditions.
Research Objective #2: Develop a predictive model of when and how ecological community reorganization can occur. Standard paleoecological theory portrays benthic marine communities as associations of organisms that track appropriate environmental conditions. However, current ecological theory suggests that communities can experience fundamental reorganization in response to environmental perturbation events. Documenting whether fossil communities have experienced radical reorganization (research objective #1), and if they have, understanding the conditions under which such change occurred (research objective #2) are critical to making use of paleontological information to understand ancient depositional environments. A modeling approach has been undertaken to understand the conditions under which reorganization occurs. Using parameters derived from Permian Basic brachiopod data, the model suggests that these kinds of communities are expected to remain at a stable equilibrium as long as environments remain stable or change gradually and continuously, even over 1-10 million years. However, when disrupted (for example, by a change in sea level corresponding to a sequence bounding unconformity that was fast enough to prevent species from tracking their optimal habitats), the communities return to a distinctly different equilibrium. This finding provides an explanation for the kind of community change documented as part of research objective #1 that is not predicted by standard theory.
Results. In terms of sequence stratigraphy, these findings imply that biotic gradients identified during one stable period (third-order sequence) may not be informative in another. However, they also provide a means of testing whether a proposed stratigraphic event was local or affected an entire region: Large-scale (i.e., global or continental) flooding or exposure events are expected to leave an ecological signature. In contrast, faunas responding to gradual environmental change or sudden change due to more local events (e.g., tectonically induced uplift or subsidence in a region) would be expected to respond by tracking their environments rather than by reorganization.
Impact. ACS-PRF funding has made it possible to carry out two seasons of fieldwork and support a graduate student (Leigh Fall, PhD candidate), who is carrying out this research. She has presented two posters and one talk based on this research at national meetings (including winner of the “Paleontological Society Best Student Poster” award at the Geological Society of America conference in 2007) and will begin publishing her results in the next year. ACS-PRF funding also helped support a paper that will appear in the January 2008 issue of Palaios and another that is currently being revised for Nature.
