Cycle Anatomy and Variability in the Storm‐Dominated Type Cincinnatian (Upper Ordovician): Coming to Grips With Cycle Delineation and Genesis

Abstract

Although parasequence and sequence are scale-independent terms, they are frequently applied only to specific scales of cycles. For example, meter-scale cycles are commonly assumed to be parasequences or PACs. In the Upper Ordovician Kope and Fairview Formations of northern Kentucky, we examined a succession of 50 meter-scale cycles that have been variously interpreted as deepening-upward, shallowing-upward, or showing no relationship with water depth. Our analysis shows that these cycles, characterized by shifts in storm-bed proximality, are highly variable in their thickness and internal construction. Most cycles are best considered high-frequency sequences, because deepening-upward intervals are common, and many cycles contain evidence of abrupt basinward shifts in facies as expected at sequence boundaries. A minority fit the parasequence model of shallowing-upward cycles bounded by flooding surfaces. Larger, 20 m scale cycles are defined by systematic thickening and thinning trends of meter-scale cycles. However, meter-scale cycles do not display any systematic trends in cycle anatomy as a function of position within the 20 m cycles or position within the Kope and Fairview Formations. The high cycle variability and the lack of systematic stratigraphic organization with respect to longer-term cyclicity reflect either the irregularity of relative sea-level changes, the poor recording of sea-level changes in this deep-water setting, or the generation of these cycles by climate-induced cyclicity in storm intensity. These three mechanisms would generate similar patterns at the outcrop scale, so it is not possible at the present to distinguish between them.

Eustatic and tectonic control on the distribution of marine seismites: examples from the Upper Ordovician of Kentucky, USA

“…Results of new correlations in the RocklandianMaysvillian strata throughout the Kentucky region (Tobin, 1982;Holland, 1993;Holland et al, 1997;Jennette and Pryor, 1993;Dattilo, 1996;McLaughlin and Brett, 2000;McLaughlin et al, in press;Brett and Algeo, 2001;Brett et al, 2000 provide strong evidence for the persistence of certain distinctive decameter-to meter-scale limestone beds and bed bundles or cycles (see Fig. 13A,B).…”

Visual identification and quantification of Milankovitch climate cycles in outcrop: an example from the Upper Ordovician Kope Formation, Northern Kentucky

“…The Kope is composed primarily of cyclic alternations of shales/mudstones and bundles of thin (3-10 cm) beds of skeletal (brachiopod, bryozoan and crinoidal debris) packstones and grainstones, 0.5-10 m (Jennette & Pryor 1993;Holland et al 1997). Lithostratigraphic and biostratigraphic studies of the Kope show detailed correlations among closely spaced sections (Tobin & Pryor 1981;Tobin 1982;Jennette & Pryor 1993;Holland et al 1997;Brett & Algeo 2001a, b, c Bulk, low-field, mass specific magnetic susceptibility (x)…”

“…We chose the Upper Ordovician Kope Formation exposures in northern Kentucky (Fig. 1) because the sequence, cyclicity and biostratigraphy are well documented (Holland et al 1997(Holland et al , 2001aBrett & Algeo 2001c;Brett et al 2003Brett et al , 2008. Our initial attempts to correlate among outcrop sections were successful (Ellwood et al 2007a).…”

Evaluating Validity and Reliability in High‐Resolution Stratigraphic Analysis

“…Limestones of the Kope and Bellview formations were deposited on a shallow storm-dominated mixed siliciclastic-carbonate ramp. These rocks are part of the well-known Cincinnatian Series and have been studied extensively by a number of workers (Tobin & Pryor, 1981;Jennette & Pryor, 1993;Holland et al, 1997). The Kope and Bellview formations are well-suited for photographic analysis because outcrops along the AlexandriaAshland highway present a series of horizontally bedded exposures that are readily measured and photographed at high resolution with minimal outcrop step-back.…”

“…Drummond & Sheets (2001) identifi ed the presence of stratigraphic organization within the Kope composite reference section of Holland et al (1997) in the form of stratigraphic clustering of grainstone and packstone beds. As illustrated in Fig.…”