Distribution and abundance of benthic marine taxa in shelf margin depositional sequences: San Andres Formation (Guadalupian, Permian), Last Chance Canyon, New Mexico

Open Access
Author:
Brown, Garett Mattingly
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 13, 2016
Committee Members:
  • Mark E. Patzkowsky, Thesis Advisor
  • Elizabeth A. Hajek, Committee Member
  • Michael A. Arthur, Committee Member
Keywords:
  • shellbeds
  • biofacies
  • stratigraphic archiecture
  • condensation
  • Permian
Abstract:
The processes that control stratigraphic architecture also play an important role in the distribution and accumulation of fossil material. For example, abrupt changes in fossil occurrences and abundances occur at stratigraphic surfaces marking abrupt shifts in lithofacies and in stratigraphic intervals created by slow net accumulation of sediment. However, the exact controls governing the relationship between sequence stratigraphy and the distribution and accumulation of fossils are not fully understood. The well exposed and well-defined sequence stratigraphic framework of the fossiliferous Middle Permian San Andres Formation within Last Chance Canyon provides an opportunity to test these controls. This study examines the fossiliferous shell beds of the San Andres Formation in Last Chance Canyon through two depositional sequences about 180 m in thickness across approximately 700 meters along depositional dip to characterize the distribution and accumulation of fossil material. Major changes in biofacies from molluscan dominated assemblages in a channelized pelloidal sandstone to brachiopod-sponge-echinoid dominated assemblages in a heavily bioturbated sandstone occur at the maximum flooding surface. In addition, high density shell beds accumulated near the maximum flooding surface. While this implies the sedimentation rates are influencing the abundance of these fossiliferous beds, sedimentary structures indicate transport is the dominant factor within these facies. An additional biofacies and shell density change from brachiopod-sponge dominated sandstones to very dense Parafusulina dominated carbonates occurs within the depositional sequence. However, no major stratigraphic surfaces were observed at this transition, most likely indicating a shift in environments favorable to Parafusulina. These biofacies changes and high shell densities result from sediment transport or changes in productivity, rather than the processes that control the stratigraphic architecture of Last Chance Canyon.