Title

Lamination in recent Bahamian subtidal stromatolites: Origin and lithification

Date of Award

1993

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Marine Geology and Geophysics

First Committee Member

Robert N. Ginsburg, Committee Chair

Abstract

Living, lithified, subtidal stromatolites are presently growing on an oolitic sand bar in Adderly Channel, Lee Stocking Island, Exuma Islands, Bahamas. Lamination in these stromatolites consists of couplets of millimeter-scale sediment-rich, cement-poor laminae and thinner micritic laminae. This lamination is produced from a combination of the interaction of a single cyanobacterium, Schizothrix n. sp. with the sedimentary environment and the precipitation of a fragile, early framework cement associated with the Schizothrix mat. Sediment stress, caused by entrainment of grains by rapid tidal currents, excludes most other organisms from living on the stromatolite surfaces. The phototactic response of Schizothrix to shading by newly added sediment effectively binds grains which adhere to the mat surface. Definition of laminae occurs when the amount of sediment supplied to the mat decreases during periodic depositional hiatii. With little sediment accumulating on the mat, the mat is able to thicken and a lamina of micritic cement precipitates above a sediment-rich lamina either within or on the surface of the mat.Variations in the amount of sediment stress as a dune crest migrates past the stromatolites allows a succession of communities to colonize the surfaces. The addition of some organisms, diatoms in particular, when the stromatolite sits exposed in a dune trough and sediment stress is lower, appears to "dilute" the ability of Schizothrix to form a tight network of filaments and effectively trap and bind sediment in a firm mat. These organisms can trap sediment loosely but do not form a laminated fabric. Boring and encrusting organisms also present can modify the original laminated fabric produced by the Schizothrix mats. As a dune crest migrates towards a stromatolite, the amount of sediment entrained by the currents near the top of the stromatolite is increased and the only organism present which can adjust to this stress is Schizothrix.The association of cement laminae with the Schizothrix mat indicates that the mat plays some role in the precipitation of the cement. The exact mechanism for precipitation is not known, however, if the mat did not play a role, isopachous cements typically found in submarine hardgrounds would be expected. In laminated sections of stromatolites, little to no isopachous cement occurs and the thin cement laminae lithify the fine sand and silt. Only pockets of coarser, unlaminated sediment interpreted to be pocket-fills, are lithified by isopachous cements. This secondary cement as well as encrusting eucaryotes aid in the lithification of the stromatolites.The thickness of laminae within lithified portions of the stromatolites are remarkably consistent. This consistency suggests that although subtle changes in the depositional environment may cause the formation of laminae, the organisms tapping and binding sediment may control the thickness of laminae.

Keywords

Geology

Link to Full Text

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