EXPERIMENTAL AND FIELD EXAMPLES OF SUBAQUEOUS SYNSEDIMENTARY CRACKS IN SEDIMENTS

Our laboratory models produce structures remarkably similar to synsedimentary cracks and other deformation features found in ancient sedimentary rocks (both clastic and carbonate). In particular, models produce conspicuous mud cracks that result from neither desiccation nor synaeresis. Structures are generated by subjecting layers of contrasting competence (cohesive versus loose) to shear and loading. Flexible containers were filled with water-saturated sand encasing a layer of unset plaster of paris 1 - 3 cm-thick. Containers were subjected to simple shear and periodic loading while the plaster solidified (progressively: viscous, cohesive to firm) and the surrounding sand remained saturated and loose. These materials and conditions simulate shallow substratal (< 1 m) sand and compacted mud subjected to synsedimentary stresses. Cross-sections through models reveal several features identical to those found in ancient sediments where plaster=mud, e.g., contorted plaster layers with ragged upper and/or lower surfaces, delicately cuspate-lobate plaster-sand interfaces, unfilled and sand-filled cracks cutting through plaster from both above and below, plaster boudinage, and delamination of plaster at the plaster-sand interface. Such features are the "diastasis" of Cowan and James (1992). Persistent shear/loading in these experiments caused chunks (clasts) of plaster to rise to the surface through the overlying sand. "Risen" clasts resemble an intraclast lag, but experienced no current transport or winnowing! Possible sources of shear/loading in sedimentary environments include storms, seismicity and differential loading by large bedforms. Most common, perhaps, would be the rapid application of stress induced by the passage of large storm waves. Storm waves are known to transmit shear and loading several meters into the sediment pile, i.e., to substratal depths consistent with the amount of compaction inferred in these experiments (Terzaghi 1940). Such wave-induced stresses would be common across a range of water depths from the beach to SWWB. For comparison with model results, we present examples of carbonate and clastic rocks from Cambro-Ordovician strata across North America.