Abstract for 1995 Penrose Conference:
	"Mesozoic Evolution of the Cordilleran Continental Margin
		in Central and Southern California"


     Orphaned Conglomerate Cobbles of the Late Cretaceous-Paleogene
		Gualala Basin:  Still Seeking a Home.

	Schott, Ronald C. and Johnson, Clark M., Dept. of Geology and
	   Geophysics, Univ. of Wisconsin-Madison, Madison, WI  53706

Recent palinspastic reconstructions of the San Andreas Fault Zone (SAFZ)
(e.g. Powell, 1994 in GSA Memoir 178) restore the Gualala Basin, currently
located 180 km northwest of San Francisco, 465km southeastward to a
pre-Tertiary position west of the southern Sierra Nevada Batholith (SSNB)
and northeast of, and adjacent to, the restored Salinian Block.  If such
reconstructions are correct then Late Cretaceous-Paleogene conglomerates
should represent a window into the eroded upper levels of this segment of
the batholith.
	Conglomerate clasts of the Gualala Basin (GB) range in composition
from gabbros and diorites to granitoids and rhyolites, and range in size up
to three meters in diameter.  Division of the cobble suite by depositional
age and cobble composition suggests a small number of distinctive source
components.
	Cobbles deposited during the Late Cretaceous appear to be derived
from two distinct source terranes.  Gabbroic cobbles have a distinctively
oceanic isotopic signature (87Sr/86Srinitial  of 0.7026 to 0.7048, and
eNd(t) of +6.1 to +9.2) and Late Jurassic ages (140-165 Ma).  Potential
parents for the gabbroic cobbles are only sparsely exposed in the western
Sierra Nevada today, and are nowhere recognized in the Salinian Block.
Interstratified granitic and rhyolitic cobbles thus far yield
87Sr/86Srinitial>0.706,  eNd(t) of -6.6 to +1.3, and mid-Cretaceous ages
(93-101 Ma, 120 Ma).  The felsic cobble suite is geochemically and
isotopically similar to rocks currently exposed in the southern Sierra
Nevada and northern Salinian Block, but textural evidence suggests
derivation from a shallower level of the batholith.  Cobbles from the
Eocene section at Gualala are restricted to granodioritic to tonalitic and
trondhjemitic compositions.  Garnet-bearing tonalites and trondhjemites
have unusual REE patterns and Early Cretaceous ages (129-140 Ma) which are
rare anywhere in the Cordilleran batholiths of California and are not
currently recognized in either the southern Sierra Nevada or Salinian
Block.