Silurian and Lower Devonian Basin and Basin-Slope Limestones Copenhagen Canyon, Nevada
Jonathan C. Matti, Michael A. Murphy, Stanley C. Finney, 1975. "Silurian and Lower Devonian Basin and Basin-Slope Limestones Copenhagen Canyon, Nevada", Silurian and Lower Devonian Basin and Basin-Slope Limestones Copenhagen Canyon, Nevada, Jonathan C. Matti, Michael A. Murphy, Stanley C. Finney
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Silurian and Lower Devonian carbonate rocks in central Nevada are grouped into two distinct, time-equivalent assemblages: (1) an eastern dolomite suite, deposited on a broad, peritidal carbonate platform; and (2) a western limestone-clastic suite, deposited in subtidal, predominantly basinal environments oceanward from the margin of the dolomite suite platform.
Silurian through Lower Devonian (Llandoverian through Pragian) limestones belonging to the limestone-clastic suite occur at Copenhagen Canyon, Nevada. This sequence is conformable and is divided into three formations, which include, in ascending order, the Roberts Mountains Formation, the Windmill Limestone, and the Rabbit Hill Limestone.
The Silurian–Lower Devonian rocks are predominantly graptolitic, evenly laminated, argillaceous lime mudstone and wackestone interpreted as basin and basin-slope deposits. Graded and nongraded bioclastic limestone beds carrying fragmented shoal-water benthic faunas are interbedded with the mudstone-wackestone sequence. The bioclastic sediment is interpreted as allochthonous calcareous sand (allodapic packstone and grainstone) that was probably deposited by turbidity currents. Deposition by grain-flow and debris-flow mechanisms may also have occurred.
The Roberts Mountains Formation and the Windmill Limestone are interpreted as distal, level-bottom sequences. They are western equivalents of the Lone Mountain Dolomite, a pervasively dolomitized shoal-water carbonate complex that accumulated east of Copenhagen Canyon on the margin of the dolomite suite platform. Allodapic sand in the Roberts Mountains Formation and in the Windmill Limestone was derived from this eastern carbonate build-up.
The lower member of the Rabbit Hill Limestone is a distal, level-bottom sequence and is a western equivalent of the lower Kobeh Member of the McColley Canyon Formation. The upper member of the Rabbit Hill Limestone is a proximal sequence deposited on the basin slope, as indicated by thick allodapic sand and soft-sediment slump structures. Proximal basin-slope conditions in the upper member resulted from basinal progradation of the Kobeh Member carbonate-platform complex during deposition of the upper Rabbit Hill member. Thus, the upper Rabbit Hill member in the limestone-clastic suite is time-correlative with the lower Kobeh Member in the dolomite suite, but it was subsequently overlain by the prograding Kobeh Member.
Spinoplasia Zone brachiopods occur in the upper Rabbit Hill member. The Spinoplasia Zone and the Trematospira Subzone were previously thought to be entirely superpositional in relation. However, we believe the two brachiopod assemblages are facies-controlled, largely coeval benthic communities.
Figures & Tables
- carbonate rocks
- environmental analysis
- Lower Devonian
- Middle Silurian
- Roberts Mountains Formation
- sedimentary rocks
- United States
- Rabbit Hill Limestone
- Rabbit Hill
- Copenhagen Canyon
- Windmill Limestone
- basin slopes