Analysis of fossil communities in the Del Rio Formation, Upper Cretaceous, Texas




Hinote, Russell Elliot

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Six fossil communities were recognized in the Del Rio Formation based on Q-mode cluster analysis of 158 samples collected from 14 Del Rio measured sections in central through north Texas. Using this method of analysis, samples were clustered into six distinct groups on the basis of the faunal similarity of fossil assemblages contained in the samples. Each group of samples contained a set of recurrent fossil assemblages, and the taxa in each set represented the composition of a fossil community. The six Del Rio communities maintain consistent stratigraphic positions in the formation throughout the area. The vertical succession of fossil communities corresponds to lithologic variations in the formation which represent changes of environmental conditions that occurred during deposition. Examination of the relative abundance, morphological features, and inferred living habits of the taxa in the six fossil communities aided interpretation of the environmental conditions which controlled the composition and distribution of each community. The classification of gross community feeding habits and substrate niche preferences gave additional clues to environmental conditions, and the relative diversity of taxa in the communities gave an indication of environmental favorability. Environmental changes occurring during the deposition of the Del Rio were interpreted to have resulted from a minor marine regression which probably began during deposition of the upper Georgetown Limestone. Community A in the basal Del Rio reflects an unstable environment caused by the influx of terrigenous clay at the beginning of Del Rio deposition. Community B, dominated by epifaunal, suspension-feeding taxa with adaptations for survival on a soft substrate, indicates high influx of clay, shallowing of water, and gradual restriction of open marine circulation during deposition of the lower Del Rio. Maximum regression probably occurred during the deposition of the middle Del Rio. Community C indicates restricted conditions were greatest at this time and substrate fluidity was high. Community D in the upper Del Rio reflects a gradual transgression which resulted in deeper water and more open circulation of water. However, the presence of many taxa with adaptations for living on a soft substrate indicates clay influx was still relatively high. Supply of clay diminished and became episodic at the close of deposition of the Del Rio. The restricted, soft-substrate fauna of Community E, which lived during periods of clay influx, alternates with the more diverse fauna of Community F, which lived during times of little or no clay influx. Deposition of the Del Rio ended with the last influx of clay.