Progressive Diagenesis in Quaternary to Late Tertiary Carbonate Sediments: Sequence and Time Scale

Abstract

ABSTRACT Two different sequences of progressive diagenesis and lithification in carbonate sediments were studied: one a sequence deposited under subaerial conditions (eolianites) and the other a sequence deposited under intertidal marine conditions (beach rocks). The diagenesis of the eolianites in subaerial conditions can be related to the following stratigraphic sequence: Stage I--Recent sediments; Stage II--Late Glacial sediments (dated by 14C at 7000 to 10,000 years); Stage III--Ultimate Interglacial sediments (probably 80,000 to 100,000 years); and Stage IV--Penultimate Interglacial (probably 130,000 to 170,000 years), Ante-Penultimate Interglacial (probably 200,000 to 300,000 years), and Neogene sediments (2 million to 25 million years). In the eolianites four diagenetic stages are recognized. Stage I consisted of the original unconsolidated skeletal grains. In Stage II these grains were cemented near grain-to-grain boundaries by an early calcite derived from outside by percolating water; interparticle pore space was largely preserved. Quartz and other silicate grains were partially replaced by calcite. Magnesium was removed from Mg-calcite to yield calcite without textural change. This led to a sharp reduction in the magnesium and manganese concentrations. Almost no dissolution of aragonite occurred at Stage II. Isotopic exchange led to enrichment in the lighter carbon and oxygen isotopes; the strontium and uranium concentration decreased. In Stage III the interparticle pore space was entirely occluded by a drusy cal ite cement. Skeletal particles of aragonite were dissolved out, forming molds now filled with a drusy calcite. The fabrics of the carbonate rocks studied became stabilized. Dissolved aragonite grains served as a local source of calcite cement. Meteoric water caused further enrichment in the lighter carbon and oxygen isotopes. The isotopic composition became stabilized at this stage. Magnesium and manganese were further depleted by removal from calcite; magnesium became stabilized and probably manganese, too. Strontium and uranium experienced further reduction; uranium became stabilized. No major changes occurred from Stage III to Stage IV except for the coralline algal fragments becoming lighter in color under the microscope, indicating loss of organic matter. A second generation calcite replaced the calcite which formed in Stages II and III, but was observed only in the Penultimate Interglacial sediments. Strontium decreased slightly to become stabilized; all other elements became stabilized in Stage III. The textural, mineralogical, and geochemical data indicate that the major diagenetic changes were completed in Stage III, prior to the Ultimate Interglacial age (80,000 to 100,000 years). Of the major mineral components, Mg-calcite had changed to calcite in In the intertidal marine environment two stages are recognized, Stage I consisting of unconsolidated carbonate sands and Stage II consisting of consolidated beachrocks. Fibrous and cryptocrystalline cement of aragonite and Mg-calcite are responsible for the change from Stage I to Stage II. The magnesium, strontium, and manganese content of beachrocks is less than that of carbonate sands; the cement is depleted with respect to these elements in beachrocks in comparison with the carbonate sands.