2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 1
Presentation Time: 8:00 AM-12:00 PM


TWELKER, Eric, 10430 Dock Street, Juneau, AK 99801, twelker.eric@gmail.com

Since the 1970s it has been recognized that cleavages, including crenulations, are often the result of pressure solution—a process favored in more porous and permeable rocks and not restricted to high pressure or temperature conditions. At the same time, it has been acknowledged that bedding plane foliations are commonly the result of diagenesis. And experimentalists have shown that pressure solution in a weak shear stress setting can result in wholesale recrystallization and create intense foliation and crenulation. The implications of this are that recrystallized and intensely foliated rocks may be formed during diagenesis in close proximity to soft and partly lithified sediment. In this poster, I review these developments and propose that in a shear stress scenario, such as sea floor collapse, a continuum of processes beginning at the sea floor with gravity induced slumping, subsurface soft sediment folds, subsurface sediment remobilization, various slips and folds in partially lithified rock, finally moving to rocks foliated, folded, crenulated and spaced cleaved by pressure solution processes.

I have applied the idea of a continuum to understand the structure at some ore deposits in Southeast, Alaska. The Greens Creek VMS deposit and the AJ and Treadwell gold deposits are centered on “bulls-eyes” of relatively intense apparently multiphase deformation surrounded by mostly undeformed rocks. Deformation is more intense lower in the stratigraphic sections. Within the deposits, fragile depositional textures and even fossils are found close to highly foliated and folded phyllites. Clastic dikes can cut the foliation. Apparently simultaneous ductile and brittle deformation are observed. Traditional structural-metamorphic explanations call for high P-T ductile deformation that is inconsistent with the observed textures. Interpretation as a continuum of soft sediment deformation to penecontemporaneous foliation in a shear stress regime resolves the inconsistencies and makes the deposits' structure easily understandable as the result of progressive deformation in a seafloor collapse.

An important implication of this theory is that measurement of structural features related to a collapse that is genetically related to ore deposits may be usable to model collapses and locate undiscovered ore deposits.