Abstract: Carbon and Oxygen Isotopic Compositions as Indicators of Cementation History in Holocene and Pleistocene Carbonate Sediments

M. Magaritz

Carbon and oxygen isotopes have been used as indicators of the origin of carbonate in Holocene and Pleistocene sediments. Study of 75 samples of calcareous sandstone from nearshore and beach zones in the eastern Mediterranean Sea reveals several environments of cementation and carbonate precipitation. Most of the carbonate in the top 0.5 m in the nearshore zone is detrital (^dgr¹⁸O = -3.7; ^dgr¹³O = -1.2 to -4.1). In one area finely crystalline 5 mal%-Mg-calcite (^dgr¹⁸O = -2.5, ^dgr¹³O = + 1.4) cement indicates in-situ cementation in pore water at temperatures of 25 to 35°C. In Holocene beachrocks, where the carbonates consist of a mixture of biogenic aragonite and cement of high-Mg calcite (5 to 10%), the aragonitic end m mber has an isotopic composition of ^dgr¹⁸O = 1.2 and ^dgr¹³C = -1.2, and the calcitic cement ^dgr¹⁸O = -3.5 and ^dgr¹³C = -1. These data suggest that the calcitic cement formed from seawater at high temperatures (41°C; such temperatures were measured in the tidal zone in this region) or from brackish waters. In places in Holocene-cemented calcareous sandstone the biogenic aragonitic fraction yields a ^dgr¹⁸O value of +1 and a ^dgr¹³C of +2.5, and the cryptocrystalline to microsparite calcite cement a ^dgr¹⁸O of -5.5 and a ^dgr¹³C of -5 indicating a percolating meteoric-water origin with part of the carbon coming from local dissolution of marine carbonate.

In the calcareous sandstone of Pleistocene age, two kinds of cement were found: (1) a sparry calcite with a ^dgr¹³C range between -8.2 and -10, and (2) micritic matrix and sparry calcite with a ^dgr¹³C range between -4.7 and -7.2. These values suggest a freshwater origin for the first group and a marine origin, at least for the carbon, in the second group.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC