THE PICKHANDLE AND JACKHAMMER FORMATIONS IN THE NORTHERN CALICO MOUNTAINS, CA: EVIDENCE OF SYNEXTENSIONAL DEPOSITION AND VOLCANISM IN THE CENTRAL MOJAVE METAMORPHIC CORE COMPLEX

The early Miocene Pickhandle Formation of the central Mojave Desert, CA is inferred to represent synextensional supradetachment basin deposits of the central Mojave metamorphic core complex (CMMCC). However, direct stratigraphic evidence of synextensional deposition of these rocks is not well documented. This study in the northern Calico Mountains, located on the hanging wall block of the Waterman Hills detachment fault in the CMMCC, provides evidence of synextensional deposition of the Pickhandle Formation and the underlying Jackhammer Formation during upper plate extension. The Jackhammer Formation is deposited on nonmylonitic pre-Tertiary basement rocks of the hanging wall block, and consists of interbedded reworked tuff and lapilli tuff, tuffaceous sandstone, conglomeratic sandstone, and localized basement-derived megabreccia, lacustrine limestone, and mafic lavas. In the east, a ~130 m-thick crystal-rich welded ignimbrite, herein named the “Mammut ignimbrite”, dominates the Jackhammer Formation deposits. The Pickhandle Formation is conformable with the Jackhammer Formation, consisting of two members: a synvolcanic lower member of monomict dacitic volcaniclastic breccia and block and ash flows, and an upper member of polymict conglomeratic sandstone, conglomerate, tuffaceous sandstone, and reworked tuff and lapilli tuff. Reddish porphyritic silicic plugs and lavas were emplaced during and after deposition of the upper Pickhandle Formation. Both of these formations represent coeval volcanic and alluvial deposits that appear to thicken and coarsen eastward toward a high-angle NW-trending, SW-dipping basin-bounding normal fault. Paleocurrent indicators suggest primarily WSW-directed flow, with clasts mainly derived from nonmylonitic basement and intrabasinal dacitic rocks. Fanning bedding dips, intraformational angular unconformities, and intercalated alluvial and lacustrine deposits are located adjacent to the basin-bounding fault; several additional NW-trending normal faults in the study area were also active during and following deposition of these rock units. Many faults exhibit postextensional strike-slip reactivation, likely related to subsequent development of the right-lateral Calico Fault system located ~7 km to the southwest of the study area.