Paleoenvironmental analysis of coastal marsh deposits in the Aguja Formation, late Cretaceous, Trans-Pecos Texas
Upper Cretaceous carbonaceous shale, lignite, and coal deposits of the Aguja Fomatlon are exposed on the southwest Â£lank of Rattlesnake Mountain in western Big Bend National Park. These deposits represent accumulation in delta plain marshes and swamps. Variations in depositional environment ranging from prograding delta plain marsh sediments to well-drained swamp deposits are distinguished among the lower, middle, and upper shale members of the Aguja.
Llthologic analysis, clay mineralogy, total organic carbon analysis, stable carbon and oxygen isotopic analysis, thin section petrography, and palynology are employed to differentiate facies. On the basis of this facies analysis prograding delta plain marsh environments are interpreted for the lower and middle shale members. The upper shale member is proposed to represent deposition in a well-drained swamp environment. Lithologic variations and a relationship between kaolinite and total organic carbon indicates cyclicity in the lower and middle shale members.
The lower shale member overlies the progradational basal sandstone member of the Aguja, and represents deposition in a delta plain marsh with crude cyclicity apparent in the facies found. Lignitic layers cap the marsh cycles, and kaolinite enrichment is found below the lignites.
The middle shale member overlies the transgressive Rattlesnake Mountain sandstone member, and is similar in lithology to the lower shale member. The middle shale member consists of carbonaceous shale and lignite, and may represent marsh progradation over the Rattlesnake Mountain sandstone.
The upper shale member overlies the Terlingua Creek sandstone member, and represents a departure from marsh deposition. Ijn situ stumps, logs, and sparse organic material in the shale indicate deposition in a well drained swamp, perhaps in a proximal deltaic environment.