Flat pebble conglomerates consisting of rounded clasts of bioclastic siltstone, sandstone and grainstone are abundant in a 233 foot (71 m) core from an Upper Cambrian sequence in the northern Powder River Basin. Flat pebble conglomerates form 20 percent of a sequence dominated by ripple-laminated and horizontal parallel-laminated lenticular beds of bioclastic siltstone, sandstone and grainstone.
Most flat pebble conglomerate beds are 1 to 7 cm thick, but some multistory units are up to 60 cm thick. Most conglomerates display weak to strong clast imbrication, and several beds exhibit bimodal imbrications. The flat pebbles are derived by segmentation of interbedded thin shale and bioclastic siltstone, sandstone and grainstone beds. The segmentation is produced by subvertical dewatering channels along which relatively fine sediment is injected. In addition to dewatering structures, other features in the finer grained deposits which indicate rapid sedimentation are load casts, piled ripples, ball and pillow structures, microfaults, and climbing ripples.
The character of the clastic component in the conglomerate matrix changes abruptly midway through the core, suggesting a major change in the nature of clastic input. Conglomerate beds in the lower part of the core contain subangular silt to very coarse quartz brought to the coastline by fluvial processes. Younger conglomerates contain smaller amounts of silt- to very fine sand-sized quartz, which most likely was introduced by aeolian processes.
The conglomerates are interpreted to have been deposited below wave base by tidal currents in a shallow marine basin. Work by Palmer (1971), Aitken (1978) and others has shown this basin was separated from the open ocean to the west by a peritidal bank. Glauconite is a minor to abundant component throughout the Upper Cambrian sequence. Its relative abundance in a sequence of rapidly deposited sediments behind a shelf-edge bank may reflect increased rates of glauconite development in the Cambrian. The increase in rate may been favored by lower temperatures and widespread anoxic conditions in the oceans during the Cambrian.
The much greater abundance of flat pebble conglomerates during the Cambrian and Lower Ordovician may be attributable to changes in biologic evolution, as proposed by Sepkoski (1982), or to an increased tidal range during this time period, or to both of these. Available evidence bearing on the tidal range in the Cambrian is sparse and inconclusive.
