About This Item
- Full text of this item is not available.
- Abstract PDFAbstract PDF(no subscription required)
Share This Item
The AAPG/Datapages Combined Publications Database
The Lower Cretaceous Edwards Limestone of west-central Texas was deposited under shallow-marine, moderate-energy shelf conditions. Exposed insular areas were developed in response to remnant structural features combined with biologic and normal sedimentologic processes (sand banks and rudist reefs). The beach complex was developed in association with an insular area thought to be a rudist reef. The reef itself is not exposed due to Holocene erosion. This carbonate sequence is a direct ancient analog of Holocene clastic beach sequences which have been described by Bernard, Leblanc, and Majors.
The beach sequence, as exposed in a series of quarries at Round Mountain, Comanche County, Texas, consists of the following zones, from top to bottom: subaerial backshore, foreshore, upper offshore, and lower offshore.
The exposed backshore consists of a complex set of subaerial sedimentary environments: subaerial crust, paleosoil, supratidal dolomite, and washover deposits from the beach. The foreshore consists of a well-sorted rudist carbonate grainstone. The dominant sedimentary structures are parallel laminations and well-developed accretion bedding. The allochems have been leached and infilled by sparry calcite or silica. The upper offshore consists of a poorly sorted rudist carbonate packstone. This zone is characterized by poorly developed accretion bedding and localized festoon cross-bedding oriented perpendicular to beach accretion. Inversion textures are the dominant diagenetic fabric of the facies. The lower offshore consists of an echinoid pellet carbonate packstone. The dominant sedimen ary characteristics of this facies are burrows, localized festoon cross-bedding, and abundance of micrite matrix.
The main points of similarity between this ancient carbonate beach sequence and Holocene clastic beach sequences are a general downward decrease in grain size and the sedimentary structure sequence from inclined laminations and beach accretion packages at the top to festoon cross-bedding and organic burrows at the base.
The main points of difference are the grain types present, particularly the abundance of biologically derived material in the carbonate beach, the preservation of the exposed backshore environments, and the complex diagenetic history of the carbonate beach which is a result of the original metastable mineral constituents.
The Cretaceous carbonate beach sequence is a further verification of the wide applicability of the conceptual process model as an interpretive sedimentologic tool regardless of sediment origin or mineralogy.
End_of_Article - Last_Page 1787------------