ABSTRACT

Many ancient carbonate rocks consist of dolomite interlayered with limestone on a centimeter or millimeter scale. Many such rocks contain cryptalgal laminites or stromatolites. Comparable Recent stromatolitic and flat laminated algal sediments are composed of alternating layers of particulate carbonate sediment and algal mats. The characteristics of algal-rich and sediment-rich laminae are identical to those of the dolomite and calcite laminae, respectively, of ancient forms. On this basis, dolomite laminae in ancient interlaminated sediments are considered equivalent to, and hence derived from, algal-rich lamina in Recent stromatolitic sediments; calcite laminae are derived from sediment-rich laminae. Modern algal sediments of this type, however, are not known to contain thin layers f dolomite.

It is suggested that the dolomite layers are secondary. During deposition, magnesium ions are complexed organically in the algal mat layers. The algal sheath material, in which the magnesium is concentrated, is very stable and does not decompose until long after deposition, or even lithification. Only when the organic matter is decomposed is the magnesium released to form dolomite in the micro-environment of the relict algal mat layers. Thus the dolomite, although secondary, conforms to the primary algal mat layers of the sediment.

Laboratory experiments support the hypothesis. Sheath material of the major stromatolite-forming blue-green alga Schizothrix calcicola grown in sea water shows a three to four fold increase in Mg/Ca ratio relative to that in the sea water medium. Sufficient magnesium is complexed in a single 2 mm thick algal mat layer to produce a layer of dolomite 1 mm thick. No dolomite was precipitated in the algal mat layers experimentally, but crystallization of 17-20 mole percent-Mg calcite in contact with the sheath material was achieved, showing the influence of the organic layer micro-environment on the composition of the carbonate mineral produced. The actual formation of dolomite may be a long term process, probably achieved by the partial replacement of the carbonate matrix in conta t with the relict algal mat layers, and/or by selective replacement of high-Mg calcite within the algal mat layers.

The hypothesis explains why thinly interlayered dolomite is not found abundantly in Recent sediments, and yet why the dolomite layers so faithfully conform to the primary sedimentary structures in ancient rocks. The hypothesis refers specifically to laminated cryptalgal rocks, but should be considered for other problematical types of ancient dolomite where the distribution of dolomite may be related to the original distribution of organic matter in the sediment.