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Lower Cretaceous rudist reefs control facies distribution in the subsurface Edwards and Stuart City Formations in South Texas. An outcropping rudist reef of nearly equivalent age in Mexico, and the Florida reef tract—Florida Bay Recent model, facilitate definition of the subsurface facies. Dominant facies in the Edwards and equivalent units include burrowed and algal mudstone, skeletal siltite, skeletal calcarenite, rudist reef, and planktonic foraminiferal carbonate mudstone. Comparing the Cretaceous and Recent models, a change in reef frame from rudists to corals is the principal difference but minor faunal components in back-reef sediments are similar.

Rock samples are described quantitatively and compared vectorially. A reduction in the dimensions of the vector space is accomplished by factor analysis. Sample composition of the reef and associated facies is determined from the resulting rotated factor matrix. A factor score, computed by post-multiplying the transpose of the standardized data matrix by the square of the rotated factor matrix, emphasizes important rock components controlling the various facies. Thus, the number of critical components needed to outline the environments is reduced.

Parameters for the analysis include components modified by textural and structural adjectives (excluding burrowed carbonate mudstone). A second factor analysis was run using only important faunal components as outlined by the factor score. Environments outlined by the two analyses are very similar notwithstanding this reduction in the number of descriptive parameters. However, micro-sedimentary structures and textures are important in environmental interpretation of facies containing extinct faunas.

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