Reports: G8 46096-G8: Refining Sequence Stratigraphic Interpretations Using Benthic Fossil Communities from the Permian Basin of West Texas

Thomas D. Olszewski, Texas A&M University
Researcher story

The aim of this project was to use ecological theory to enhance sequence stratigraphic interpretation.  The central hypothesis was that ecological systems can accommodate small environmental changes without disruption of their community structure, whereas larger environmental changes lead to reorganization of community membership and species abundances.  Classical paleoecological theory portrays benthic marine communities as stable associations of organisms that track environments.  However, modern ecological theory suggests that, in addition to tracking preferred habitats, communities can experience fundamental reorganization in response to environmental change.  The rationale for undertaking this research was that determining the nature of community reorganization in response to environmental perturbations – for example, sea level changes associated with sequence stratigraphic surfaces – would allow more reliable interpretation of changes in depositional environments, thereby enhancing sequence stratigraphic interpretations.

ACS-PRF funds made it possible to support my first PhD student, Leigh Fall.  She received a research assistantship for one full year and money to carry out approximately fifteen weeks of fieldwork over three summers as well as purchase necessary laboratory materials.  The ACS-PRF grant helped to establish a healthy research program in stratigraphic paleobiology, a factor in the choice of the PI as recipient of the Paleontological Society's Schuchert Award in 2009.

This research took advantage of the well-known silicified brachiopod faunas of the Permian Basin exposed in Guadalupe Mountains National Park.  The first research task was to establish an environmental and sequence stratigraphic context for brachiopod-bearing deposits focusing on the Capitanian Bell Canyon Formation (Fig. 1).  This unit is composed of a series of named "carbonate tongues" composed of breccias and grainstones separated by quartz sandstones.  The carbonates, interpreted as debris flows, grain flows, and turbidites, are the source of silicified fossils.  Sections were measured and sampled through the Pinery, Rader, Lamar, and Reef Trail Members from various locations in the park, including areas that had never been sampled for brachiopods before.  Multiple locations insured that the same range of brachiopod habitats was sampled in each carbonate unit.

Previous sequence stratigraphic interpretation of this interval established a hierarchical framework: each carbonate tongue represents the transgressive portion of a high-frequency depositional sequence with the intervening sandstones representing the late highstand and lowstand.  These high-frequency sequences stack into larger-scale composite sequences.  Previous work has identified composite sequence boundaries above the Pinery and Rader.

Forty-five samples yielded 6,038 brachiopods from fifty-three genera.  These data were used to analyze the taxonomic composition, abundance structure, and ecological heterogeneity for each of the four carbonate tongues.  Multivariate ordination and clustering revealed that the Pinery and Rader (the lower two tongues) were indistinguishable in terms of taxonomic composition and associations.  In contrast, the Lamar was very different: although many of the taxa from the Pinery and Rader remained in Lamar communities, a number of previously rare taxa became much more common.  In addition, two invasive species previously known only from southeast Asia first appear in the Lamar.  The Reef Trail collections had some similarities to the Lamar and some similarities with the Pinery and Rader.

Abundance distributions characterize the structure of ecological communities without reference to taxon identities.  In the Pinery and Rader, a distinct set of dominant taxa is separated by a sharp drop in abundance from a different set of rare taxa.  In contrast, the Lamar and Reef Trail show a much more continuous distribution of abundances without a distinct separation between dominant and rare classes.

Diversity partitioning is a means of analyzing how taxa are distributed among locations on an ecological landscape.  In this case, the communities of the Lamar were found be less heterogeneous – i.e., much less different from one another – than those of the Pinery and Rader, which were statistically indistinguishable from one another.  The Reef Trail shows the highest degree of diversity partitioning: relative to the earlier carbonate tongues, its communities at different locations were very different from each other.

Overall, the pattern of brachiopod communities is one of consistency in taxonomic composition, abundance structure, and landscape heterogeneity between the Pinery and the Rader, suggesting that these communities represent a steady state that was not affected by environmental change associated with the sequence boundary between these two units.  In contrast, Permian Basin brachiopod communities were not resilient enough to survive a larger disruption between the Rader and Lamar without fundamental reorganization of their composition and structure.  The Reef Trail community appears to be on a path returning it to pre-Lamar abundance structure but with a different cast of taxa in dominant roles.  However, before it could return to a new steady state, the Permian Basin became a closed, evaporitic system, destroying all brachiopod habitats; its high landscape heterogeneity may be a precursor to this terminal event.

This research provides insight into the nature of ecological community response to environmental disruption: these brachiopod communities appear to be resilient to environmental fluctuations up to a threshold event size, but are disrupted and reorganized during larger fluctuations.  Return to previous states is not instantaneous and can take enough time that it can be resolved stratigraphically.  Importantly, ecological reorganization in response to disruption does not require termination of lineages – i.e., in this case, biological systems were able to respond to base level change without experiencing mass extinction.

From a geological perspective, these results indicate that ecological responses can be used to assess the relative magnitude of sequence stratigraphic surfaces.  In this case, the lack of community disruption indicates that the base level change associated with the Pinery-Rader boundary was of lesser magnitude than the event between the Rader and Lamar.  Constraining the magnitude of sequence bounding surfaces places an important constraint on possible basin-scale correlations, thereby providing greater understanding of the stratigraphic architecture of the Permian Basin, a major hydrocarbon province in North America.

Figure 1. Study Interval.  Dark gray indicates Permian reef crest, stippling indicates carbonate tongues, heavy lines indicate composite sequence boundaries, light lines indicate high-frequency sequences boundaries, "CS" stands for "composite sequence". (Modified from Tinker, 1998.)

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