Eastern Section Meeting

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Facies patterns and stratigraphy of the Cambrian Trempealeau Formation, Huron County, Ohio


Strata of the Sauk Megasequence are not exposed at the surface in Ohio, however they are important petroleum reservoirs, and often form paleogeomorphic traps such as up-dip pinchouts and isolated paleotopographic remnants beneath the Knox Unconformity. Core from the Conroy Well #2, in Huron County, Ohio, penetrates the Knox Unconformity and dolostones and siltstones of the “B-zone” of the Trempealeau Formation. The core has been described and samples thin sectioned and tested for porosity and permeability. Six lithofacies are identified incuding: 1) laminated glauconite rich dolomitic quartz siltstone; 2) laminated dolomitic quartz siltstone; 3) dolorudstone; 4) silty, burrowed dolomudstone; 5) thrombolitic dolomudstone; and 6) stromatolitic dolomudstone. The first three facies are interpreted as hummocky cross stratified open shelf storm deposits that accumulated below fair weather wave base. The glauconite likely accumulated during starved, deeper water periods, during the flood-portion of a high frequency cycle, and then mixed into laminated and cross laminated silts during storm events. The latter three facies are interpreted as shallow subtidal to intertidal deposition. Strata occur in a pattern of eight shallowing upward parasequences separated by erosional surfaces interpreted as karst truncations. Each parasequence shallows upward and contains several but not all facies. Parasequences within the lower portion of the core are dominated by HCS facies and the upper half are dominated by shallow facies. The overall shallowing upward trend indicates a progradational parasequence set that is part of a highstand systems tract that likely belongs to one of several third order sequences within the Sauk 3A Supersequence. More detailed biostratigraphy could further delineate which third order sequence, barring missing beats or amalgamation of sequences at this position high on the platform. Porosity and permeability is controlled by dissolution of the thrombolitic dolomudstone along the Knox unconformity, and perhaps also associated with the inter-parasequence karst surfaces. Preliminary observations indicate dolomitization occurred very early post-depositionally, prior to karsting. Much of the vuggy porosity is occluded by later pyrite cement.