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Trace-Fossil and Storm-Deposit Relationships of San Carlos Formation, West Texas

Cheryl L. Metz, Sheila P. Bednarski

Two distinct assemblages of trace fossils are preserved in the storm deposits in delta-front facies of the Upper Cretaceous San Carlos Formation, west Texas. The assemblages represent two widely differing responses to storm deposition and sediment-trace-fossil relationships, indicating that other environmental parameters, probably water depth and oxygen levels, influenced trace-fossil distribution within the San Carlos delta front. Evidence of the storm-deposited nature of the sandstones includes a scoured basal contact, planar to hummocky cross-stratification, and a upper contact that is either ripple marked or is gradational with overlying shales.

Trace-fossil assemblage 1 contains a large, previously undescribed Chondrites-like form as the dominant trace fossil. It is found only in the top of the storm sandstones. The singularity of this Chondrites-like trace fossil and the similarity of the interbedded shales with the underlying basinal to prodeltaic shales suggest that similar dysaerobic conditions existed part way up the delta front. Conversely, trace-fossil assemblage 2 contains Ophiomorpha, Thalassinoides, Planolites, Chondrites, and Trichichnus associated with the storm sandstones. Thalassinoides is also abundant in the overlying shales. Primary depositional features of the storm sandstones may be masked by bioturbation. An increase in the amount of carbonaceous material and basal shell lag associated with individual storm sandstones is evident in comparison to assemblage 1. This increase, along with diversity of enclosed trace fossils and degree of bioturbation, indicates deposition in shallower, more oxygenated waters than assemblage 1. It is inferred that sandstones of assemblage 2 were deposited in closer proximity to a distributary mouth, alleviating the dysaerobic conditions affecting assemblage 1.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.