ABSTRACT

Carbonate-cemented sandstone from two shallowly buried sources and one surface exposure were examined to characterize the texture and composition of early, shallow cements in sandstones. Pebbles from the storm beach of Padre Island, derived from upper Pleistocene deposits exposed on the shelf, are sands cemented by microcrystalline low-Mg calcite (caliche), by rare high-Mg calcite, and/or by a rind of dolomite. Sandstone from dredge spoils of the Intracoastal Waterway (12 ft [4 m] deep) in Laguna Madre is cemented by microcrystalline low-Mg calcite (caliche), spar, dolomite, gypsum, and rare aragonite and barite. Sandstone from the Ingleside barrier is cemented by spar only.

Calichified sand displays layered crusts, pedotubules, pisolites, and dispersed sand grains. Calcite is luminescent, contains <0.5 mole percent Fe and Mn and 500 ppm Sr, and forms crude rhombs 1 to 4µm long. ¹³C values are -8 to -5 (PDB), and ¹⁸O values are -4 to -2 (PDB). Spar occurs in clean sands and coquinas. It is low-Mg, contains little Fe or Mn and <500 ppm Sr, and does not luminesce. ¹³C values are from -8 to -2 , and ¹⁸O values from -4 to -2 . ¹⁴C dating of spar yields anomalously young ages. Dolomite occurs in clean sands and overprints caliche as a rind 15 µm thick of rhombs and spheroidal grains. A few caliche samples are dolomitized. Dolomite contains 30 to 47 mole % Mg and up to 800 ppm Sr and occurs as both disordered and ordered phases. ¹³C values of dolomite range from -15 to -4 and ¹⁸O from +3 to +4 .

Spar formed in upper Pleistocene beach sands where Laguna Madre presently stands when meteoric water recycled carbonate from aragonite and high-Mg calcite shells. Caliche formed over a broad subaerially exposed area during the late Pleistocene lowstand. Aragonite and high-Mg calcite 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 years. The lighter carbon isotopic values attest to a significant contribution of organic carbon in the cements. Nannobacteria and biofilms occur within many carbonate phases, suggesting that microbes aided in the carbonate precipitation process.

Shelly limestone contains the same cements as does the sandstone, plus an odd low-Mg calcite with depleted carbon that appears to be a syndepositional cement.