C H Jones, CIRES, Univ. Co.-Boulder, Boulder, CO 80309-0216; e-mail cjones@mantle.colorado.edu
L J Sonder, Dept. Earth Sci., Dartmouth College, Hanover, NH 03775; e-mail leslie.sonder@dartmouth.edu
As part of our ongoing paleomagnetic studies in the Lake Mead region of Nevada we sampled four stratigraphic sections at the base of the middle Miocene age Horse Spring Formation (17.5-13.5 Ma). These sections are separated by the northwest-trending, right- lateral Las Vegas Valley shear zone (LVVSZ). Several lithologically unique units of regional extent and tectonic importance occur in these sections and their temporal correlatuion is important for studies of tectonic rotations and extension. Samples were demagnetized by low-field AF and thermal demagnetization. A magnetic component interpreted as the primary magnetization was generally present between 375 and 575¡C. Although the rocks of the lower Horse Spring Formation in this area are mostly nondistinctive silts, sands, and gravels, several distinctive, coarser conglomerates and breccias occur in laterally continuous, similar stratigraphic positions throughout the region. Polarity patterns are similar in all four sections with mostly reversed polarity samples punctuated by short normal polarity intervals. A basal conglomerate composed of well- rounded pebble to cobble size clasts of Paleozoic and Mesozoic lithologies lies slightly unconformably upon Mesozoic units. One or two similar conglomerates occur higher in the section. There is nou distinctive polarity interval to constrain the basal conglomerate. The higher conglomerates do not occur in consistent polarity intervals, indicating time transgression of deposition or varying deposition intervals. In the Frenchman Mt. area south of the LVVSZ lenses of monolithologic breccia with little or no matrix are present higher in the section. Lenses of the Gold Butte rapakivi granite are particularly noticeable. North of the LVVSZ in the Gale Hills a debris flow containing almost solely rapakivi granite clasts occurs in a similar stratigraphic position. Both rapakivi granite bearing layers occur in similar positions in a normal polarity interval suggesting synchronous deposition. This provides a unique marker bed across the LVVSZ for other studies of vertical axis rotation, for estimates of Cenozoic extension, and reconstruction of the Horse Spring Basin.