UTILIZATION OF GAMMA-RAY PROFILES FOR ANALYZING MEGAFLOOD TURBIDITES

Turbidity currents are hyperpycnal flows that are driven by density. Quaternary megafloods drained the continents as turbidity flows in multiple episodes at multiple locations. Previous studies of megaflood-turbidite deposits incorporated methods of indirect seismology and detailed sedimentology. The megaflood-turbidites were discriminated from other turbidites based on their inferred seismic reflection sequences in submarine channels and fans, their stratigraphy, and radiometric dates. The pioneering and assiduous efforts were spatially limited, however, because the investigations dealt with a few of Missoula turbidite sequences.

We employed a new method that incorporates the efficacy of gamma-ray profiles in detecting megaflood-turbidites. Also, the method expands the spatial scope such that turbidites from any megaflood can be studied. Consistent patterns exist in the gamma-ray deflection profiles, in which shalely strata are overlain by terrigenous sands, and the patterns correspond horizontally to one another and to sedimentary sequences previously published. Major channels contain the thickest sandy strata, whereas locations peripheral to the channels maintain thinner sandy caps. The majority of gamma-ray sequence profiles located at or close to the submarine channels suggests that the turbidities were restricted by the channels; although others were carved into channels. Published reports suggest the Deep Hurd within the English Channel is an example. Continental shelves, such as the New Jersey shelf, had no submarine channels and consequently no megaflood turbidites formed. Although Greenland and Iceland produced glacial outbursts, no appreciable volume of sediment was transported. Presumably the volume of sediment is related to the size of the source area. Therefore, gamma-ray logs, which are affordable and accecible, are efficient in detecting megaflood turbidites, comparable to previous seismic-sedimentologic methods. Additionally, our method permits a broader spatial range for identification of megaflood turbidites, because the spatial limit hinges only on the accessibility of gamma-ray data acquired from the worldwide ocean drilling research programs.