Bedform development in carbonate skeletal sand : influence of fluid velocity and grain size in flume experiments

Sedimentological processes responsible for the development of current-derived bedforms and stratification types in carbonate sand, and their potential application to the interpretation of the origin of carbonate rocks with clastic textures, have been poorly understood because of a historical lack of experimental research. Flume studies using carbonate skeletal sand transported by unidirectional flows replicate the progression of quartz sand bedforms well-documented by previous workers. Transport experiments were performed on three different size fractions: 2ø (0.25 mm diameter), 1ø (0.50 mm diameter), and a heterogeneous natural mixture of sizes ranging from 3ø to -1.5ø (0.12-2.50 mm diameter), with a mean size of 0.56 mm. The boundary fields for carbonate bedform phases developed at lower threshold velocities than for equivalent-sized quartz sand. This is attributable to variation in shape and high Internal porosity of the skeletal grains, which contrast with idealized equant quartz grains used to develop existing models for bedform stability. The sequences of carbonate bedforms that developed with increasing flow velocities are: 2ø - small → ripples → dunes → upper plane bed → antidunes; 1ø - lower plane bed → dunes → upper plane bed → antidunes; mix - small ripples → dunes → upper plane bed → antidunes. Cross-stratification, horizontal lamination, and size-grading produced by bedform migration were recognizable at both a macro- and micro-scale. Carbonate mud was generated under upper flow conditions by the abrasive removal of adhering and pore-filling sediment particles, from micrite envelopes on grain surfaces, and from primary skeletal material. Thin-section and SEM observations additionally suggest rounding of grain surfaces due to mechanical abrasion. Desiccated micrite layers formed micrite intraclasts, which, under low flow regime conditions, maintained their integrity and were incorporated Into bedform stratification, indicating a high preservation potential.