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Abstract: Shallow Calcite-, Aragonite-, and Dolomite-Cemented Sandstones, South Texas Gulf Coast

Earle F. McBride, Hiromi Honda

Carbonate-cemented sandstones from two shallowly buried sources were examined to characterize the texture and composition of early, shallow cements in sandstones. Pebbles from the storm beach (source 1) derived from late Pleistocene deposits exposed on the shelf are sands cemented by microcrystalline calcite (caliche) and/or by a rind of dolomite. Sandstones from dredge spoils of the Intercoastal Waterway (4 m deep) in Laguna Madre (source 2) are cemented by microcrystalline calcite (caliche), spar, dolomite, gypsum, and rare aragonite and barite.

Calichified sands display layered crusts, pedotubules, pisolites, and dispersed sand grains. Calcite is low-Mg, non-luminescent,

contains < 0.5 mol % Fe and Mn and 500 ppm Sr, and forms crude rhombs 1 to 4 µm long. ^dgr13C values are -5 to -4% (PDB) and ^dgr18O values are -4 to -1 (PDB). Spar occurs in clean sands and coquinas and is not overprinted by dolomite. It is low Mg, contains no Fe or Mn and <500 ppm Sr, and does not luminesce. ^dgr13C values are from -8 to -2% and ^dgr18O values from -4 to -2%. 14C ages of spar range from 32,000 to 38,000 yr. Dolomite occurs in clean sands and overprints caliche as a rind 15 microns thick of rhombs and botryoidal grains. A few caliche samples are dolomitized. Dolomite contains 30 to 47 mol % Mg and up to 800 ppm Sr, and occurs as both disordered and ordered phases. ^dgr13C values of dolomite ange from -15 to -4% and ^dgr18O from +2 to +4%.

Spar formed in late Pleistocene beach sands where Laguna Madre presently stands when meteoric water recycled carbonate from aragonite shells. Caliche formed over a broad subaerially exposed area during the late Pleistocene low stand. Aragonite cement formed during the Holocene sea level rise. Dolomite formed by the replacement of calcite (caliche) and as a direct precipitate during seepage refluxion when Laguna Madre became hypersaline, possibly in the last 200 yr. The lighter carbon isotopic values attest to a significant contribution of organic carbon in the cements. Nannobacteria occur within most carbonate phases suggesting they aided in the carbonate precipitation process.

AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994