Nature and Causes of Transgressive and Regressive Cycles in a Lower Paleozoic Quartzose Sheet Sand

Jim Mazzullo

The stratigraphic section in the interior of North America contains a series of lower Paleozoic quartzose sheet sandstones, and their classic representative is the St. Peter Sandstone. The St. Peter was deposited during a Middle Ordovician transgression, but it is far thicker than recent transgressive shelf sands. Thus it has been hypothesized that the formation was deposited during repeated transgressions and regressions of the continental interior.

A newly excavated section of the St. Peter Sandstone in the Twin Cities basin of Minnesota supports this hypothesis and bears evidence for the nature and causes of transgressions and regressions during its accumulation. The section exposes two intervals of fine to very fine-grained, thick-bedded, massive, bioturbated sandstone which were deposited during transgression of shelf environments. They are separated by an interval of medium to fine-grained sandstone with multiple mutually truncating scours, tabular and trough cross-beds (including herringbones) with shale drapes on their foresets, wavy ripples, shale rip-up clasts, and minor bioturbation traces, which were deposited in a tide-dominated coastal sand-flat environment which prograded onto the open shelf. This regressive deposit is laterally discontinuous over a distance of 4 km, which suggests that the transgressions and regressions in the Twin Cities basin were caused by variations in the rate of sediment supply.

Petrographic analysis indicates that these variations in the rate of sediment supply were themselves controlled by the mode of the sediment supply, and particularly the relative contribution of wind-blown and stream-borne sand to the margins of the basin. When the wind was the principal mode of sand supply, coastal environments were confined to the margins of the basin because the load of the wind was diffused along the shoreline. When streams were the principal modes of sand supply, coastal environments, which formed at their mouths, prograded toward the center of the basin.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.