Stratigraphic Completeness of Carbonate-Dominated Records From Continental Interiors Versus Continental Margins: Stratigraphic Thinning Occurs Via Condensation and Omission At Multiple Scales

Abstract

Over Phanerozoic time scales, stratigraphic records from continental interiors, or cratons, are dramatically thinner and are generally assumed to be relatively incomplete, with more numerous and longer-duration hiatuses, compared to records from more rapidly subsiding continental margins. However, this assumption need not be true for the shorter time scales (i.e., several 10 6 years and shorter) over which accumulation of the preserved stratigraphic record on the continental interior actually takes place. This study compares carbonate-dominated sedimentary records from the Devonian continental interior (Iowa) and continental margin (Nevada) to evaluate whether the continental-interior record is (1) miniaturized, i.e., thinner, but equally complete; (2) comparable in thickness and quality where the sedimentary record is preserved, but certain portions of the record are notably absent, i.e., omitted or truncated; or (3) so invariably preserved that equivalent stratigraphic packages cannot be recognized compared to the continental-margin record.This study finds that fourth-to fifth-order cycles in Iowa are, on average, approximately half as thick and half as numerous compared to the continental margin of Nevada. Moreover, the stratigraphic record on the continental margin of Nevada is dominated by cycles that are composed of tidal-flat and shallow subtidal lithofacies, capped by simple flooding surfaces, and exhibit catch-up or keep-up depositional styles. The continental-interior record in Iowa is dominated by cycles that are composed of subtidal lithofacies, capped by a range of bounding surfaces, and exhibit catch-down or give-up depositional styles. These findings suggest that the thin record in Iowa is largely a result of low sedimentation rates associated with suppressed carbonate production in this epeiric-sea setting, rather than minimal subsidence rates and an increased potential for subaerial exposure and associated loss of record.

Middle to Upper Devonian Skeletal Concentrations From Carbonate-Dominated Settings of North America: Investigating the Effects of Bioclast Input and Burial Rates Across Multiple Temporal and Spatial Scales

“…epicontinental seaway. The stratigraphic records in Iowa and Nevada preserve analogous depositional environments ranging from a shallow inner 'shelf' to deeper middle 'shelf ' to basinal outer 'shelf ' (as defined by Witzke and Bunker 1996; see also Johnson et al 1985;Witzke and Bunker 1989;Day et al 1996;LaMaskin and Elrick 1997;Kaufmann 2006;Brady 2015). The present study does not include purely basinal or tidal flat deposits, but instead highlights the bioclast-rich facies defined and described in Brady (2015): restricted shallow subtidal, shallow subtidal, intermediate subtidal, and deep subtidal.…”

“…In total, 162 skeletal concentrations from Nevada and 129 concentrations from Iowa were assigned to restricted shallow subtidal, shallow subtidal, intermediate subtidal, and deep subtidal lithofacies ( Table 1, described in Brady 2015). Taxonomic, stratigraphic, and taphonomic features of the skeletal concentrations were described in the field and verified with a subset of polished slabs from skeletal concentrations.…”

“…Thickness.-Despite significantly thicker stratigraphic packages (at the scale of facies, meter-scale cycles, and third-order depositional sequences) in Nevada compared to Iowa (Brady 2015), thickness distributions of skeletal concentrations in Nevada and Iowa are quite similar (Fig. 5A, 5B).…”

“…Specifically, thick (. 30 cm) and densely packed brachiopod shell beds typically are hosted in shallow subtidal facies, which tend to occur at the tops of meter-scale cycles in both regions (Brady 2015). Where associated with meter-scale cycle bounding surfaces, skeletal concentrations in Nevada and Iowa tend to occur immediately below, rather than above these discontinuity surfaces.…”

“…The relatively thin MiddleUpper Devonian record in cratonic Iowa reflects omission of entire meterscale cycles: the preserved cycles are relatively complete (have a similar number of facies), but they are on average half as thick as those on the more rapidly subsiding margin of Nevada, implying lower sedimentation rates at the scale of depositional cycles compared to stratigraphic units on the continental margin (Brady 2015).…”