THE CORRELATION AND TECTONIC IMPORTANCE OF EARLIEST CRETACEOUS, SHALLOW MARINE, CHERT-RICH, QUARTZOSE CLASTICS CAPPING JURASSIC ARC ROCK IN THE NORTHERN SIERRA NEVADA, CALIFORNIA AND CARSON RANGE OF WESTERN NEVADA

The Evans Peak sequence (EPS) of Taylorsville, CA includes shallow marine, fossiliferous black shale, chert-pebble conglomerate, limonitic quartzose arenite, and minor limestone grading upward into fossiliferous (plants and dinosaur bone) fluvial clastics and silicic tuff of Barremian (U-Pb) age. The EPS sits unconformably atop Callovian – Tithonian, felsic volcanic rock, and coarse-grained, locally-derived, fossiliferous clastics of the Mount Jura sequence (MJS). 80-km SE of these EPS exposures, a very similar succession of qtz-ss, chert pebble conglomerate and black shale grading upward into coarse clastics and silicic tuff is present in the Genoa Peak pendant (GPP) of the Carson Range, Nevada. Here, the clastics overlie an Oxfordian basement of felsic to silicic volcanics and feldspathic ss and shale.

For this study, qtz-ss of the GPP was sampled to assess potential correlation with the EPS. The resulting data allow for an earliest Berriasian depositional age of 144.0 ± 3.4 Ma (n=150). This age is comparable to the Berriasian age (n=164) determined for the EPS. Both the EPS and GPP DZ samples contain a local Mesozoic arc basement component (MJS in the EPS sample; Pine Nut terrane in the GPP sample) that has been mixed with older, pre-Mesozoic recycled material derived from the east. Precambrian components of the EPS include clusters representing the Ouachita-Appalachian region augmented by SW Laurentian sources including erg sand. Similar age clusters are present in the GPP sample. These similarities, and comparable ages of the underlying arc basement suggest the qtz-rich clastics of the GPP and lower portion of EPS near Taylorsville are earliest Cretaceous lateral correlatives. More importantly, these shallow marine successions require that the Jurassic arc basement rapidly ‘collapsed’ at the close of the Jurassic allowing shallow marine burial by cratonward-derived, 'extra-basinal' sands. It is unlikely that this represents a ‘local’ event as preliminary mapping in the White Mtns of SE CA suggests a similar, qtz- and chert-rich section exists atop Jurassic arc rock there too. These chert-rich, shallow marine exposures east of the Sierran crest may represent a plate scale response (e.g., rapid extension) to evolving tectonic conditions along the western edge of the Cordillera at the close of the Jurassic.