61st Annual Report on Research 2016

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. The primary objective of Year 1 was to determine the distribution and abundance of fossils in relation to depositional environments and significant stratigraphic surfaces.

Methods:

We examined fossiliferous shell beds of the San Andres Formation in Last Chance Canyon through a single fourth-order depositional sequence to characterize biofacies changes in this shelf margin succession.

Fossil occurrences and abundances were mapped onto the north face of Last Chance Canyon using a combination of measured stratigraphic sections, estimates of faunal abundance in the stratigraphic sections, GPS location of individual fossil occurrences, and faunal counts of fossiliferous beds. Associations of fossil taxa that characterize each depositional environment were determined using multivariate analysis of faunal counts, including cluster analysis and detrended correspondence analysis.

Interpretations of depositional environments were based on lithologic descriptions, sedimentary structures, facies relationships and transitions, and examination of thin sections.

Results:

Lithofacies and Environmental Interpretation

Channelized Peloidal Sandstone — This lithofacies is composed of medium (10-30 cm) to thick-bedded (30-100 cm) fossiliferous peloidal packstones grading into very-fine grained sandstone. Graded bedding, soft sediment deformation (flame structures), and meter scale cross bedding suggests rapid deposition in channelized flow. This evidence combined with observations of large-scale channels (> 10 m) suggests this facies was deposited in distal outer ramp turbidite channels and lobes (Phelps and Kerans 2007).

Bioturbated Very-Fine Sandstone — This facies is composed of massive bedded (>3 m) very fine grained sandstone with occasional thin (3-10 cm) to medium (10-30 cm ), planar laminated very fine grained sandstone beds. This facies was likely deposited below storm influence in slope and toe of slope environments. Large-scale clinoforms in this facies downlap on the channelized peloidal sandstone.

Thin Bedded Fossiliferous Sandstone — This facies is composed of thin (3-10 cm) to medium 10-30 cam) bedded very fine-grained sandstones. It overlies the bioturbated very-fine sandstone facies and reflects primarily deposition in middle slope environments.

Thin-bedded Sandy Skeletal Packstone — This facies is composed of thin-bedded (3-10 cm) sandy skeletal packstone. Skeletal grains form dense shell beds of fusulinids (Parafusulina) with rare brachiopods. This facies overlies and has a sharp or gradational contact with the thin-bedded fossiliferous sandstone. It was deposited in an upper slope environment.

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.

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. The stratigraphically higher brachiopod-sponge and fusulinid assemblages do not show evidence of transport out of habitat.

Implications:

The ordination (detrended correspondence analysis; DCA) of fossil assemblages shows a primary gradient of molluscan to fusulinid to brachiopod-sponge dominated assemblages. Given what is known about the environmental preferences of these taxa, the DCA is consistent with an interpretation that water depth is the primary control on their distribution, with bivalves and gastropods in shallow water followed by fusulinids and brachiopods and sponges in the deepest water. However, because the depositional environments in Last Chance Canyon are dominated by gravity-driven transport, biofacies variation in this shelf margin succession reflects primarily fossil transport in a turbidite channel and fan complex, rather than simply water depth.

The molluscan biofacies is transported from the shelf and is now found in the deep-water carbonate turbidite channel and fan complex. The fusulinid biofacies is found essentially in place high in the San Andres, but fusulinids and echinoids were transported down slope and form the fusulinid-echinoid biofacies. The brachiopod-sponge biofacies shows little evidence of transport in the slope environment.

Future Work:

In year 1 of this grant, Garett Brown completed his MS thesis on this project. His thesis is entitled “Distribution and abundance of benthic marine taxa in shelf margin depositional sequences of the San Andres Formation, Last Chance Canyon, New Mexico.” Garett presented two abstracts at the Geological Society of America meetings in 2015 and 2016 on his preliminary results and he is now preparing a manuscript for publication. Future work will focus on his observations on shell beds, fossil density, and the distribution and occurrence of chert in relation to stratigraphic architecture. Upcoming fieldwork will include using a drone to photograph cliff faces that are difficult to access. This information will be used to build 3D models of facies relationships and channel morphologies using structure-for-motion methods.