Efficacy of gamma-ray profiles for analyzing megaflood turbidites

Abstract:

The various episodic deglacial megafloods of the Quaternary transported continental sediments into the marine domain. The terrestrial sediments exited the continents as hyperpycnal flows and were deposited on the ocean floor as turbidity currents. Previous investigations of pomegaflood-derived turbidites used sedimentologic and seismic methods. The studies entailed the discernment of megaflood-turbidites from those of other causes by delineating their submarine locations, lithology and radiometric ages, and by inferring sequences from seismic reflection profiles. Such scrupulous work, however, was limited in spatial extent because the analyses pertained only to a few sequence profiles from the Missoula-flood turbidites.

We have extended the effort by demonstrating the efficacy of gamma-ray profiles as proxies for detecting the megaflood-derived turbidites, which also greatly augment the spatial extent and allows for various megaflood-turbidite surveys. Typically, gamma-ray deflection patterns consistently reveal thick terrestrial sandy beds capping older marine shalely beds, which are laterally traceable to lithologic sequences analyzed in previous studies. The thicknesses of the sandy caps depend on their proximity to main submarine channels. A number of gamma-ray profiles situated at or near submarine channels suggest that the flows were confined whereas a minority of them had even carved their own channels (e.g., in the English Channel), as consistently inferred from the literature. Despite the numerous jolkulhlaups generated, less sediment originated from Greenland and Iceland as a result of their less erodible landmasses. Also predictably, gamma-ray profiles from continental shelves (e.g., the New Jersey shelf) lack such megaflood turbidites as they had been subaerial during the Quaternary glacial times. Thus, utilization of gamma-ray logs allows the detection of megaflood turbidite sequences in a more accessible and affordable manner than did the sedimentologic-seismic methods. Further, our research widens the spatial scope, thereby enabling surveys of megaflood turbidites globally as the spatial extent is limited only by the availability of gamma-ray profiles from various ocean drilling programs.