PALEOGEOGRAPHIC IMPLICATIONS OF SHALLOW MARINE TO SUBAERIAL, LOWER CRETACEOUS ROCKS IN THE EASTERN MESOZOIC BELT OF NORTHEASTERN CALIFORNIA NEAR TAYLORSVILLE

The Lower Cretaceous Evans Peak sequence (EPS) outcrops near Taylorsville, California and represents one of few exposures of Lower Cretaceous supracrustal rock identified in the Sierra Nevada or central and western NV. Similar, age-correlative rocks exist in the Jackson Mtns and Krum Hills of NW Nevada (King Lear Fm), Cortez Mtns and Pinon Range of central Nevada (Newark Canyon Fm), and possibly Peavine Peak, near Reno, NV, and near Oroville, CA (Monte del Oro Fm).

The Lucky S Fm of the lower EPS consists of thinly-bedded, black marine shale interbedded with well sorted, channel mouth bar, chert-pebble conglomerates, and limonitic arenite. Minor limestone and brackish-water, branchiopod bearing, back-bay siltstone are also present. Marine fossil material in the formation is broken or abraided suggesting shore-face effects. Abundant fragmentary fern pinna (< 4 cm in length) reflect river-transported material rafted out to sea. Recent DZ work suggests an Early Cretaceous (late Berriasian) onset of deposition with source areas including the underlying Upper Jurassic volcanic rock and Tithonian granite, as well as distal Ouachita-Appalachian orogen sands enriched with other SW Laurentian components. Pz microfossils in chert pebbles suggest a contribution from the Inyo Mtns area of CA. The gradationally overlying Trail Fm consists of plant-bearing, subaerial tuff, vertebrate containing, fluvial ss, shale, and volcanic cobble conglomerate. Plant fossils in the Trail Fm are up to 15 cm in size and represent cycad(?) fronds and tree fern blades. The angular, light blue or grey weathered bone fragments are commonly < 2 cm in size. The lack of rounding suggests rapid burial within aggrading, sand-rich, streams. To date, the largest bone fragments found include the proximal end of an anterior dorsal rib, broken just shy of the head and tubercle, and a possible mid-section of a neural vertebra spine. The detrital components of Trail Fm ss include a diminished contribution from non-local basement sources.

While EPS deposition began in the late Berriasian, ashflow tuffs in the upper Trail Fm have yielded Barremian zircons (127.2 ± 3.1 Ma and 129.4 ± 2.0 Ma) indicating deposition of the entire EPS spanned approximately 15 m.y. and transitioned from shallow marine to subaerial environments. Aptian granite intrudes the EPS.