Lower Cretaceous Carbonate Beaches: ABSTRACT

Abstract

Several carbonate beach sequences have been recognized and studied in Lower Cretaceous shallow-marine shelf limestones of the Edwards Formation, Central and West Texas. These sandstone bodies are local features associated with exposed insular areas developed in response to remnant structural features combined with biologic and normal sedimentologic processes (sand banks and rudist reefs). From top to bottom, 4 zones are recognized: backshore, beach foreshore, upper offshore, and lower offshore. The backshore consists of laminated, mud-cracked, fine-grained supratidal dolomite with gypsum molds and associated paleosoils. Thin washover deposits of coarse calcareous sands are common. The beach foreshore consists of a coarse rudist carbonate grainstone with a subaerial crust developed at the top. The dominant sedimentary structures are inclined laminations, accretion bedding, and vugs. Collapsed beachrock slabs are common. The upper offshore consists of poorly sorted rudist carbonate packstone with some finely interbedded coarse rudist carbonate grainstone and pellet carbonate packstone. This zone may contain festoon crossbedding trending normal to the direction of beach accretion. Beach rock cobbles derived from the beach foreshore zone above are common. The lower offshore zone consists of an echinoid-pellet carbonate packstone with common burrows and some festoon crossbedding. Useful criteria for the recognition of ancient carbonate beaches are: vertical sequence of sedimentary structures, from accretion beds at top to burrows at base; vertical sequence of texture and fabric from coarse, well-sorted grainstones at top to poorly sorted fine-grained packstones at base; presence of evaporative supratidal facies at the top of the sequence; and association of keystone vugs and beachrock slabs with the accretion-bedded part of the sequence. End_of_Article - Last_Page 641------------