DECI-METER TO M-SCALE CYCLICITY IN GLAUCONITIC SANDSTONE OF THE RENO MEMBER, LATE CAMBRIAN FRANCONIA FORMATION, SOUTHEASTERN MINNESOTA

The Late Cambrian Reno Member of the upper Franconia Formation in southeastern Minnesota is at the base of a transgressive systems tract (TST) and is conformable with overlying silty dolostone of the St. Lawrence Formation. Deci-meter to m-scale lithofacies stacking patterns in the upper part of the Reno Member near Rollingstone suggest cyclic deposition. An idealized cycle is characterized from bottom to top by: 1. glauconite-rich, medium- to fine-grained sandstone, dominated by intraclasts oriented sub-parallel to bedding; 2. fine-grained hummocky (HCS), swaley (SCS), and trough cross-stratified (TCS) sandstone dominated by vertical burrows, with fewer horizontal burrows in glauconite-rich troughs; 3. sparsely glauconitic siltstone with abundant bioturbation, including skolithus and arenicolites; and 4. a thin mud drape with abundant horizontal burrows of the zoophycus ichnofacies

A minimum of five complete fifth-order cycles in the upper part of the Reno Member has been identified by detailed cm-scale measurement of stratigraphic sections. Small-scale relative sea-level fluctuations may have produced these cycles. Each cycle began with falling sea level, together with increased sediment supply, allowing for deposition of the intraclast-bearing lithofacies near fair-weather wave base (FWWB). Subsequent sea level rise accompanied deposition of the HCS lithofacies below FWWB. Finally, continued sea-level rise permitted deposition of highly bioturbated siltstone and mudstone lacking in HCS and TCS further below FWWB. Falling sea level then initiated deposition of the overlying cycle.

An alternative interpretation of the lithofacies stacking patterns involves repetitive storm events sweeping across the shelf. Previously deposited, weakly lithified glauconitic sand and silt was scoured from the shelf by hurricane-scale storms. This sediment was re-deposited by waning return currents according to decreasing particle size. Intraclast-bearing glauconitic sand was deposited first, followed by hummocky stratified sand whose deposition was controlled by combined flow. Very fine-grained sand and silt was subsequently deposited from suspension and was draped by a thin layer of mud. Upward decrease in the scale of HCS suggests decreasing energy and increasing water depth through time.