ABSTRACT

The Lower Cretaceous Edwards Limestone of westcentral 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 in the Edwards Limestone described here was developed in association with an insular area thought to be a rudist reef. The reef itself is not exposed due to Recent erosion. This carbonate sequence is a direct ancient analogue of Recent clastic beach sequences as described by recent workers such as Bernard, Leblanc and Majors (1962) and compiled by Potter (1967).

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. These zones exhibit the following general characteristics. The exposed backshore consists of a complex set of subaerial sedimentary environments: subaerial curst, paleo soil, supratidal dolomite and wash-over deposits from the beach. The foreshore consists of a well-sorted rudist lime 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 lime 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 this facies. The lower offshore consists of an echnoid pellet lime packstone. The dominant sedimentary 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 described Recent clastic beach sequences are: a general fining downward grain size sequence, the general 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, 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.

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