COMPARING THE RELATIVE STRATIGRAPHIC COMPLETENESS OF CARBONATE SEDIMENTARY RECORDS FROM CRATONIC INTERIORS VERSUS CONTINENTAL MARGINS

Contemporaneous sedimentary records from the tectonically-stable interiors of continents vs. their subsiding margins often differ dramatically in preserved stratigraphic thickness. Consequently, stratigraphic records from the cratonic interior are generally assumed to be relatively incomplete, with more numerous gaps caused by erosion and non-deposition. Non-reefal carbonate shelf deposits from the Middle to Upper Devonian continental margin in Nevada are nearly an order of magnitude thicker than latitudinal- and time-equivalent carbonate deposits from the cratonic interior of Iowa. Nonetheless, these two records appear to be equally complete at million-year time scales based on correlations using third-order sequence stratigraphy and biostratigraphy. This study compares the two records at finer stratigraphic scales to determine the means by which the record in Iowa is condensed and/or truncated relative to the record in Nevada.

Stratigraphic sections measured by the author and previous workers were analyzed to compare the records in Iowa and Nevada at the level of facies and meter-scale (0.5 to 10 m thick) depositional cycle. The meter-scale cycles are defined by systematic facies stacking trends and their bounding discontinuities that reflect non-deposition and erosion. The cycles analyzed constitute three third-order depositional sequences (~1-2 m.y. in duration, ~25 to100 m thick) that can be correlated between Iowa and Nevada.

Meter-scale cycles in Iowa tend to be ~1.5 to 5 times thinner than their counterparts in Nevada. However, both settings have similar numbers of facies preserved in each meter-scale cycle, suggesting the record of depositional environments in Iowa is condensed but not significantly truncated relative to Nevada. Each correlative third-order depositional sequence in Iowa contains ~1.5 to 10 times fewer meter-scale cycles than those in Nevada, indicating that significant truncation may be concentrated at third-order depositional sequence boundaries and/or along meter-scale cycle bounding surfaces in Iowa. These findings imply that the meter-scale building blocks of the stratigraphic records of cratonic and continental margin carbonates are qualitatively comparable, but in cratonic settings are miniaturized and less frequently preserved.