Inversion of Bedding and Parasequence Types Preserved in Shelfal Mudstone Strata to Significant Marine Processes and their Controls of Rock-Property Variations
Mudstone strata contain an almost bewildering variety of physical, chemical, and biogenic attributes at the lamina to bed scale (mm – dm). Our observations of more than 7 km of Paleozoic to Pliocene mudstone in core and outcrop revealed ordered patterns in this variety-- repeated associations of lithofacies, bedding, sedimentary structures, and stratal stacking patterns at the bedset to parasequence scale. We quantified characteristics of each association and linked them to sets of depositional processes. Most shelfal mudstone strata appear to have accumulated in one of three end-member facies association successions (FASs) that can be related to physiographic settings and depositional regimes through characteristic modes of sediment transport, accumulation, and variations in benthic-energy and oxygen levels. FAS-1 comprises 1- to 10-meter-thick coarsening/thickening-upward stratal units, defined by lithologic indices: percent sandstone/siltstone/grainstone, maximum grain size, thickness of individual coarser bedsets. Physical sedimentary structures include scours, starved wave ripples, starved current ripples, planar-parallel beds, wave-enhanced sediment-gravity flow beds, graded beds, wave ripples, current ripples, and gutter casts. FAS-2 comprise 1- to 14-meter-thick coarsening/thickening-upward stratal units, distinguished by the common occurrence of palynodebris throughout, as well as thin lags of macrofossils and skeletal phosphate in basal portions, Bouma B-C bedsets, and soft-sediment deformation with minimal, horizontal burrows in its middle portions, and scours, graded and planar-parallel beds, current ripples, and lags in its upper portions. FAS-3 comprises 1- to 8-meter-thick stratal units that coarsen/thicken upward in their basal to medial portions, but then coarser grained beds tend to thin upward in the upper portions. Also common are flaser, wavy, or lenticular ripple beds, combined-flow ripples, and thin beds enriched in macrofossils and skeletal phosphate. We interpret these FAS's as recording accumulation on portions of continental shelves that were dominated by storm waves, river floods, or tidal currents, respectively. Each has a distinctive stacking pattern of rock properties that we summarize as a characteristic parasequence type. This approach has the potential to fully integrate insights from modern oceanographic studies into more robust interpretations of the rock record, especially of map pattern of rock properties.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014