SEQUENCE STRATIGRAPHY AND STRATINOMY OF MARINE HARDGROUNDS: EXAMPLES FROM THE MIDDLE PALEOZOIC OF EASTERN LAURENTIA

Sequence stratigraphic motif, taphonomy, and paleoecology were evaluated for ~50 hardgrounds in the middle Paleozoic (Upper Ordovician to Middle Devonian) of eastern Laurentia. Hardgrounds occur most frequently in portions of depositional sequences indicative of periods of low net sedimentation rate. They are concentrated within back-stepping, late transgressive systems tracts (TSTs), and typically represent flooding surfaces. Hardgrounds are commonly found at the contacts of limestone beds with overlying shales, which coincide with tapho- and biofacies changes suggestive of a drop in environmental energy and episodic influx of mud. Carbonate sediments were probably cemented while buried under a thin blanket of sediment. Winnowing, perhaps in response to large storm events, stripped away surficial sediments, leaving exposed hard substrate and, in some cases, undercutting or dislodging portions of the cemented surfaces. Details of taphonomy suggest that some hardgrounds were buried and exhumed multiple times. In some cases, these surfaces were modified, likely through onlap of corrosive waters, in response to sea level rise. Pyrite and phosphate crusts are also common, especially on hardgrounds that mark maximum starvation surfaces. In certain, more oxic settings, oolitic ironstones also developed on these surfaces. Such mineralization, indicates concentration of iron and phosphates due to lack of dilution and/or upwelling of nutrient-rich waters. Hardgrounds have been variably interpreted as either: a) exposure surfaces, or b) drowning unconformities. Neither of these inferences is fully correct. The first is certainly not supported by field data. The skeletal grainstone bodies that are typically capped by the hardgrounds represent deposition during sea level rise, rather than during lowstands. Furthermore, the taphonomy of hardgrounds suggests that, in many cases, carbonate production ended prior to major mud influx. Rather, hardgrounds formed in response to relative sea level rise and consequent siliciclastic sediment starvation. Hardgrounds from the Paleozoic of eastern Laurentia are most commonly encrusted by crinoid holdfasts and bryozoans, and bored by Trypanites, representing persistent low-sedimentation, hard substrate-adapted biotas that proliferated during TSTs.