Reports: ND855616-ND8: Fossil Occurrences, Associations, and Taphonomy in a Sequence Stratigraphic Framework: Testing Predictions in the San Andres Formation (Middle Permian, New Mexico)
Mark Patzkowsky, PhD, Pennsylvania State University
In order to make inferences about ecological and evolutionary processes, paleontologists must understand the sedimentary processes that allow sediments and fossils to accumulate. Of primary importance is the stratigraphic architecture of sedimentary basins, which can impart a pattern to the occurrence and abundance of fossils that reflects primarily sedimentary processes, and not ecology and evolution. Previous studies have looked at only a small number of depositional settings and there is a critical need to know whether patterns and processes from these few studies can be widely applied to other depositional settings and through time. Our primary motivation in this study is to determine how the composition, abundance, and density of macrofossils in the San Andres Formation in Last Chance Canyon, New Mexico relate to stratigraphic architecture. The San Andres Formation is a mixed carbonate-siliciclastic, shelf margin succession dominated by gravity-driven sediment transport. In year 1, we spent four weeks in the field, measuring and describing stratigraphic sections in Last Chance Canyon. We also collected fossil abundance data with field counts tied to the stratigraphic sections. Finally, we performed multivariate analysis of the fossil data to identify associations of fossil taxa and how they related to depositional environments and environmental gradient. The primary objective of Year 2 was to perform additional fieldwork to confirm interpretations and to collect targeted data needed for publication a manuscript.
We acquired thin sections from samples collected from our measured sections in Year 1. Thin sections were examined to confirm and modify facies descriptions.
Additional fieldwork was necessary to check work from the previous year and to gather additional information for publication. Additional counts of fossils on bedding planes were made to expand the dataset for the biofacies analysis. Long axes of oriented fossils (echinoid spines, fusulinids) were measured with a compass whenever possible. Orientation of elongate fossils may indicate direction of transport of fossil material.
We used a drone to take pictures of the canyon walls to aid in determining sand body geometry. We made multiple passes along designated areas of the canyon. These pictures will be used in a structure for motion analysis to obtain 3D models of channel sand bodies.
Biofacies Distribution by Environment
Cluster analysis and ordination of fossil counts identified four relatively distinct fossil assemblages (biofacies) that characterize specific lithofacies. A molluscan biofacies characterizes the channelized peloidal sandstone facies and is dominated by bivalves, gastropods, and cephalopods with a minor contribution by fusulinids. A fusulinid-echinoid biofacies is found at the base of the bioturbated very-fine sandstone facies in the toe of clinoforms that downlap on the channelized peloidal sandstone facies. Fossil abundance decreases higher in the bioturbated very fine sandstone facies where a brachiopod-sponge biofacies occurs. A fusulinid biofacies dominated by Parafusulina is found within the thin-bedded sandy skeletal packstone facies above the bioturbated very fine sandstone facies. Additional collections from Year 2 were added to the dataset and confirmed the preliminary analyses of Year 1.
Graded bedding, soft sediment deformation, disarticulated fossils, and orientation of fusulinid tests within the channelized peloidal sandstone facies and the base of the bioturbated very fine sandstone facies indicates that skeletal elements of the molluscan and fusulinid-echinoid assemblages were transported down ramp and deposited in outer ramp and basin turbidite channels and fans. Additional observations in year 2 indicate that the stratigraphically higher brachiopod-sponge and fusulinid assemblages also show evidence of transport out of habitat. Bedding plane exposures with oriented echinoid spines and fusululinid forams indicate a transport direction of 90 degrees east of north. Restoring these transport directions for Permian paleogeographic reconstructions suggests transport to the southeast (ca. 140 degrees east of north) into the Delaware Basin. Sandstone channel bodies also indicate transport of sediment downslope in the same direction into the Delaware Basin.
Downslope transport was a dominant process that determined the occurrence and accumulation of fossil remains in this shelf margin depositional setting. Fossil assemblages (biofacies) reflect primarily transport instead of ecological associations. This result is important for how paleoecologists interpret the fossil record. In most marine paleoecological studies, transport out of habitat is not a concern so that assemblage information can be interpreted as ecological associations. The demonstration of transport in the Permian of Last Chance Canyon raises a cautionary note for paleoecologists in that gradient analyses cannot always be interpreted as an ecological gradient. The good news is that out of habitat transport is relatively easy to recognize and is limited to few depositional environments. In Last Chance Canyon, this shelf margin setting spans only a few kilometers along depositional dip compared to most widespread shallow marine environments in the Permian epeiric seas. Abundant channel sand bodies that contain directionally oriented fossil fragments suggest transport from shelf settings to the shelf margin and slope. This means that stratigraphic and sedimentologic observations that demonstrate fossil transport are unambiguous, and fossil transport should be easy to identify.
We are preparing a manuscript on the role of fossil transport in the formation of Permian biofacies in Last Chance Canyon. We are working on a second project that builds on our work in Last Chance Canyon. Simple conceptual models suggest that fossiliferous deposits should accumulate where sedimentation is low. Our preliminary results suggest that variation in fossil density is driven more by transport, rather than fossils accumulation during low sediment input. We are developing a numerical model to identify the various processes that lead to fossil accumulations, which we will use to interpret our results from Last Chance Canyon. This will be, as far as we know, the first attempt to model fossil accumulation in a stratigraphic context, where sedimentation rate, productivity rate, and transport are combined into one model.