DUCTILE DEFORMATION OF WALL ROCK AT CONTACTS BETWEEN FERRAR DOLERITE SILLS AND BEACON SANDSTONE, DRY VALLEYS, ANTARCTICA: A CONSEQUENCE OF DISAGGREGATION AND RECEMENTATION IN SUPERCRITICAL FLUIDS DURING INTRUSION

The Dry Valleys of Antarctica expose sills of Jurassic Ferrar Dolerite intruding into Permian Beacon Sandstone. Many contacts, in detail are irregular in that the sandstone appears to have ductilely molded into 10 cm-deep cusps that outline finger-like or egg-carton shapes in 3-D. Two alternative hypotheses can explain cusp formation during intrusion: (1) The sandstone underwent intragranular plastic deformation; (2) The sandstone underwent microscopic disaggregation and recementation. Petrographic analysis, using both cathodoluminescence (CL) and polarized-light imagery, of a series samples cored at 5-cm increments from the contact to a distance of 30 cm above the contact, demonstrate that plastic deformation of sand grains did not occur—there is no change in grain shape as a function of distance from the contact, and grains within cusps do not display undulose extinction or deformation bands. Instead, we observed a progressive change in the volume of cement relative to grains in the sandstone as a function of proximity to the contact. Specifically, 30 cm from the contact, there is less than 10% cement, whereas immediately adjacent to the contact, there is more than 30% cement. The paragenesis of the sandstone, defined by CL, includes 7 events: deposition of grains, compaction and fracturing, growth of a lilac-colored quartz rim on grains, precipitation of a blocky quartz (black under CL), pressure solution, growth of “hairy illite" (clusters of randomly oriented flakes), and finally growth of two generations of calcite with the older cement appearing bright orange under CL and younger cement appearing yellow under CL. Hairy illite and calcite cements occur only in samples very close to the contact. These observations imply that cusp formation involved disaggregation (fluidization) of the sandstone, at a grain scale, prior to deposition of the last three cements. The last three cements recemented the sand grains to reform the rock. We suggest that during sill intrusion, heat transformed pore water into a supercritical fluid capable of dissolving preexisting cement and disaggregating the sand. Cooling of the fluid eventually allowed precipitation of new cement in new pores and cracks. In this context, cusps may be mullions, formed as a result of shortening accompanying cooling and contraction along the contact.