Geometry and Evolution of the Palisades Reef Complex, Silurian of Iowa

TLDR: The Palisades Reef Complex is asymmetrical, having an arcuate boundary on the north and west sides, and a general decrease in sediment grain-size and total thickness towards the south-east.

Abstract: Earlier sub-division of the Gower Formation into Le Claire (reef) and Anamosa (inter-reef) is inadequate and misleading, because the wide variety of lithologies demands a more elaborate and precise terminology; and because a large proportion of the Anamosa is later than the Le Claire reefs. The Palisades area provides an initial model for understanding facies relations elsewhere in Iowa, but not all Gower lithologies are represented. The Palisades Reef Complex is asymmetrical, having an arcuate boundary on the north and west sides, and a general decrease in sediment grain-size and total thickness towards the south-east. This presumably reflects a prevailing wind from the north-west. The first growth phase of the complex, represented by the crinoid-coelenterate facies, consists of a large number of crinoid mounds, which are most highly differentiated in the Marginal (windward) Zone; large, but simpler in the Main Crinoid Zone; and small or absent in the Leeward Zone. The Marginal Zone mounds are themselves asymmetrical, having a late-stage blanket of Coelenterate Beds on their windward sides only. The mounds are not wave-resistant reefs in Lowenstam's (1950) sense. The second growth phase is represented in the Marginal Zone by the "Brady facies." This consists mainly of laminated dense dolomite, but includes thick beds of rhynchonellids. The facies forms large-scale wedge-beds on the mound flanks, and is believed to have been derived from a platform on top of the earlier crinoid mounds. The Brady facies grades lithologically and geometrically into beds consisting almost entirely of laminated dolomite (Anamosa). In the Leeward Zone the second growth phase is represented solely by Anamosa dolomites. Off-reef lithologies are exposed, but difficult to correlate. Pene-contemporaneous cave deposits suggest sub-aerial exposure of the complex at about the end of the first phase of its development.