Sedimentary Controls on the Formation and Preservation of Microbial Mats in Siliciclastic Deposits: A Case Study from the Upper Neoproterozoic Nama Group, Namibia
N. NOFFKE, A. H. KNOLL, J. P. GROTZINGER
Shallow-marine, siliciclastic depositional systems are dominated by physical sedimentary processes, with penecontemporaneous cementation playing only a minor role in sediment dynamics. For this reason, microbial mats rarely form stromatolites in siliciclastic environments; instead, mats are preserved as wrinkle structures on bedding surfaces. Microbial mat signatures should be widespread in siliciclastic rocks deposited before the Cambrian Period; however, siliciclastic shelf successions of the
... upper Neoproterozoic Nudaus Formation, Nama Group, Namibia, contain only sparsely distributed wrinkle structures. The facies distribution of observed structures reflects the superposition of a taphonomic window of mat preservation on the ecological window of mat development. Mat colonization is favored by clean, fine-grained, translucent quartz sands deposited at sites where hydrodynamic flow is sufficient to sweep mud from mat surfaces but insufficient to erode biostabilized laminae. During periods of reduced water agitation, microbial baffling, trapping, and binding entrain quartz grains into mat fabrics, increasing the thickness of the living mat layer. Mat preservation is facilitated by subsequent sedimentary events that bury the microbial structures without causing erosional destruction. Pressure originating from sediment loading forms molds and casts at bedding planes, inducing the formation of wrinkle structures. In storm-influenced shelf successions of the Nudaus Formation, wrinkle structures are restricted to quartz-rich fine sandstone beds, 2-20 cm thick, that alternate with thin interlayers of sandy mud-or siltstones. Such a lithological facies developed only sporadically on the Nudaus shelf, but is common in shallow-marine siliciclastic rocks of older Neoproterozoic age exposed in the Naukluft Nappe Complex. The observed relationship between sedimentary environment and microbial mat preservation can be observed in other Proterozoic and Phanerozoic siliciclastic rocks, as well as in modern environments. This facies dependence provides a paleoenvironmental and taphonomic framework within which investigations of secular change in mat abundance must be rooted. Understanding the physical sedi-* Current address: mentary parameters that control the formation and preservation of microbial structures in siliciclastic regimes can facilitate exploration for biological signatures in early sedimentary rocks on Earth or other planets.