Desiccation cracks in siliciclastic deposits: Microbial mat-related compared to abiotic sedimentary origin

摘要

Siliciclastic sediment colonized by microbial mats yield a set of distinct sedimentary fabrics that are collectively called "mat-related structures (MRS)". In the rock record, versatile cracks are observed in biostabilized strata, but the mechanisms responsible for their formation remain debated. Microbially stabilized sediments produce desiccation cracks that serve as modern analogs for fossil microbial cracks. However, since both microbial mat shrinkage and clay shrinkage may contribute to the formation of these desiccation cracks, it is difficult to isolate the influence of the microbial mat on the resulting crack formation, distribution and morphology. To address this issue, we conducted a series of desiccation experiments that determine differences between microbially influenced desiccation cracks (i.e. biotic) and those formed in identical, but sterilized (i.e. abiotic) siliciclastic sediment. Three sediment mixtures were used: (1) very fine-sized sand, (2) mixed (ungraded) silt/clay, and (3) normally graded silt/clay. In all of the experiments, the water-rich microbial mat contracted substantially while drying, producing isolated pockets of shallow, but wide cracks, the distribution of which was controlled by heterogeneities in the mat structure and thickness variations of the mat. In the clay-poor substratum, the microbial mat was the only crack-forming mechanism, while in the clay-rich substrata (experiments 2 and 3) desiccation cracks were more strongly influenced by clay shrinkage. The abiotic clay-rich sediment produced a polygonal network of deep cracks intersecting at 90-120 degrees junctions. In the biotic clay-rich experiments, the microbial mat modified these desiccation features by withstanding crack propagation or by producing curled-up crack polygon margins. Even though a microbial mat shrinks substantially with desiccation, its cohesive nature and heterogeneous distribution prevents the formation of a regular crack network, but its shallow penetration into the sediment limits its influence on cracking. The biotic crack formation is best characterized by a heterogeneous distribution of wide, but shallow pockets of radiating cracks that do not connect to one another, while abiotic cracking is best characterized by an orthogonal network of deep, intersecting cracks. By comparing biotic and abiotic shrinkage cracks formed in identical sediment, we improve our understanding of modern desiccation features and thus strengthen our interpretation of the rock record. (C) 2016 Elsevier B.V. All rights reserved.