VOLCANIC ASH BED PRESERVATION IN EPICONTINENTAL SEAS/FORELAND BASINS

Primary factors related to ash eruption, transport and sedimentation (e.g., volume of ash erupted, proximity to source, dispersing wind strength/direction) have ultimate influence on the preservation of ash layers in the rock record. However, various post-depositional processes in epicontinental sea, foreland basin and continental shelf settings can strongly alter the primary record of waterlain volcanogenic ash. Event or background physical processes (e.g., storms; waves, currents; sediment dilution, condensation), biogenic processes (burrowing), or chemical processes (e.g., oxygen availability for burrowers; devitrification, early cementation) may lead to preservation of primary ash deposits, their resedimentation, and/or partial to complete mixing with background sediments.

Study of Devonian K-bentonites indicate that some represent more than single eruptions. Multi-layered beds, fossil layers within beds, detrital/authigenic minerals (e.g., glauconite, phosphate), subjacent hardgrounds, and irregular distribution of beds through space and time raise questions about the depositional history and preservation potential of volcanic ash in marine environments.

Recognizing the impact of physical, biological and chemical factors related to water depth and sedimentation rate, a simple conceptual model of ash preservation can be constructed for shallow marine environments. Better preservation potential for primary ash layers should occur with relatively high background sedimentation rates, common episodic burial of ash sediments, and decreased background physical and biological reworking and mixing. In the model, five significant ash preservation magnafacies are recognized: 1) a zone of very high physical (+ biological) activity (shoreface to shoal), with low preservation potential; 2a-c) carbonate, clastic or mixed carbonate-clastic zones of high to medium physical and biological activity (shelf/ramp), with intermediate preservation potential; and 3) a zone of low to no physical and biological activity (anoxic "basinal" settings), with increased preservation potential. Based on different physical and biological activity regimes, the zones provide a coarse-scale overview of ash bed preservation potential, or “ash bed stratinomy" in shallow marine settings.