--> Abstract: The Upper Ordovician Hollandale Embayment: Temporal Changes in Facies, Biota and Circulation in an Epeiric Sea, by J.A. Toni Simo, L. M. Chetel, S. R. Beyer, N. R. Emerson, B. Z. Saylor, B. S. Singer, and C. W. Byers; #90039 (2005)

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The Upper Ordovician Hollandale Embayment: Temporal Changes in Facies, Biota and Circulation in an Epeiric Sea

J.A. Toni Simo1, L. M. Chetel1, S. R. Beyer1, N. R. Emerson2, B. Z. Saylor3, B. S. Singer1, and C. W. Byers1
1 University Of Wisconsin-Madison, Madison, WI
2 University Of Wisconsin-Richland Center, Richland Center, WI
3 Case Western University, Cleveland

The Hollandale Embayment (NE Iowa), part if the larger North America Ordovician Epeiric Sea, occupies a transition zone between clastic and carbonate dominated environments. The interpreted source of the clastics and associated freshwater is the Transcontinental Arch. The carbonates form two facies belts; near the clastic belt are fine-grained carbonates with abundant hardgrounds, and to the south are grainy (often bryozoan-rich) carbonates with abundant evidences of storm reworking and bioclast abrasion. Through the Upper Ordovician, the clastic belt shrinks, and hardground abundance and availability of micritic sediment diminishes upward coinciding with stratigraphic condensation. Trends in brachiopod fauna correlate well with facies changes corroborating a close link between environments and dominant fauna communities. Correlation of K-bentonites (Deicke, Millbrig, Elkport, Dickeyville and Calmar) via apatite chemistry has provided a high-resolution stratigraphic and faunal framework for comparison of depositional environments and circulation history. Further, new 40Ar/39Ar ages for the Millbrig (449.3±0.9* Ma), Dygerts (447.1±0.9* Ma) and Riffle Hill (444.0 ±2.8* Ma) K-bentonites allow for quantification of physical, biological, and chemical variations. The lateral and vertical distribution of facies, fauna, and omission surfaces suggest that terrestrial factors in the Hollandale Embayment had the greatest influence, forcing changes from anti-estuarine to estuarine circulation and weakening the carbonate factory, producing a system that episodically was sediment limited and prone to the generation of omission surfaces.

*±2 sigma analytical uncertainty; all ages relative to 28.02 Ma Fish Canyon Sanidine.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005