DETRITAL ZIRCON CHARACTERISTICS OF THE EARLY CRETACEOUS LUCKY S FORMATION, EASTERN MESOZOIC BELT, NORTHEASTERN CALIFORNIA: IMPLICATIONS FOR DEPOSITIONAL AGE, SOURCE TERRANE(S) AND EARLY CRETACEOUS PALEOGEOGRAPHY

Three depositional successions occur in the Eastern Mesozoic Belt (EMB) near Taylorsville, CA. Two are Jurassic: a late Sinemurian–early Pliensbachian through Toarcian succession of shallow marine clastics/carbonate, high-K volcanics (180.3 +2.9 / -6.4 Ma) and Toarcian quartz-monzonite, and a late early Bajocian through Tithonian succession of coarse-grained clastics, vesicular flow rock, felsic tuff (160.4 +2.7 / -2.1 Ma), and hypabyssal breccias (161.5 +/- 3.1 and 154.8 +2.4 / -3.6 Ma) intruded by 148.4 +2.0 / -2.1 Ma andesite dikes and Tithonian granite. Siliciclastic material is not present in either Jurassic succession.

The Evans Peak (EPS) succession unconformably overlies the Jurassic arc rocks as an overlap sequence and consists of quartzose and chert-rich, shallow marine clastics which grade upward into plant and dinosaur bone bearing, coarse-grained, fluvial rock and distal facies, 127.2 +/- 3.1 and 129.4 +/- 2.0 Ma ash flow tuff. To identify the source(s) for the siliciclastic material in the EPS, detrital zircon analysis was conducted on sandstone from the lower EPS (Lucky S Fm). Eighty-two of the 164 zircon grains analyzed are Mesozoic with age peaks: ca. 171-159 Ma (Middle Jurassic) and 153-141 Ma (Late Jurassic to Early Cretaceous). These ages are consistent with derivation from the underlying Jurassic volcanics and Tithonian granites and permit an Early Cretaceous (Berriasian) maximum depositional age for the Lucky S Fm.

Of the non-Mesozoic zircon in the sample, Paleozoic (n=11) age peaks of 375 and 430 Ma (range of 360-441 Ma), and 524 Ma could reflect sources in the structurally adjacent northern Sierra terrane (NST). However, the EPS sample lacks ages representing lower Mississippian and upper Permian volcanic rock common within the NST suggesting the terrane was not a significant source for the lower EPS. Precambrian ages (n=71) from the sample include clusters at ca. 2.7, 2.0, 1.9-1.8, 1.7-1.6, 1.4-1.3, and 1.2 Ga – 970 Ma. This range of ages is consistent with derivation from transcontinental sand shed from the greater Ouachita-Appalachian orogen, enriched by southwestern Laurentian sources. The zircon data suggests the EPS depositional basin developed over a Middle - Late Jurassic arc basement in a location originally proximal to the modern southwestern US very early in the Cretaceous.