Kettle Rock terrane – Eastern Mesozoic belt
Kettle Rock terrane – Eastern Mesozoic belt
earliest Cretaceous to latest Triassic
This unit crops out in the northeastern-most Sierra Nevada, east and north of Taylorsville, as well as pendants east of the map area. We note that rocks in the Petersen Mountain area north of Reno included by Garside (1998) in the Peavine sequence (part of our unit NSTT) are more silicic (chiefly rhyodacite) and include maroon and red sedimentary beds, and we assign them to this terrane.
The Kettle Rock terrane consists of, from bottom to top, the Kettle Rock, Mount Jura, and Evans Peak sequences with variable lithologies across multiple structural blocks. The Kettle Rock and Mount Jura sequences exhibit greenschist, with local amphibolite, facies metamorphism, whereas the Evans Peak sequence displays a substantially lower prehnite-pumpellyite facies overprint.
The Kettle Rock sequence in the Mount Jura block consists mostly of very shallow marine, Early Jurassic fossiliferous clastic rock and carbonate. Silicic volcanic rocks crop out locally at the base of the sequence. Christe (2021a) cites a U-Pb age of ~206 Ma for these rocks. Fossils in the overlying rocks range from late Sinemurian to early Toarcian (Harwood, 1992). Christe (2021a) describes a hypabyssal basaltic andesite (~187 Ma) that cuts the Early Jurassic strata. Christe (2020) interprets it as a hypabyssal intrusive breccia that postdates deposition of the upper part of the sequence. In other structural blocks of this terrane, the Kettle Rock sequence is composed of proximal volcanic rock, mostly flow rock and hypabyssal intrusions, but also eruptive breccia and tuff breccia. Christe (2020) points out that Th/Ta ratios suggest these volcanic rocks formed in a continental margin arc. Flow-banded plagioclase porphyry from this block has yielded a zircon U-Pb age of ~180 Ma (Christe, 2020), somewhat younger than the pre-~187 Ma sedimentary rock at the top of the sequence in the Mount Jura block and the intrusive breccia found there. The volcanics are intruded by a number of stocks, one of which (the quartz monzonite Lights Creek stock) has yielded zircon U-Pb ages of ~178 and ~184 Ma (Christe and Dilles, 2014), roughly coeval and part of the same eruptive system as the metavolcanic wall rock. Christe (2020) notes that the base of the volcanics locally is a shear zone with remnants of sedimentary and tuffaceous rock similar to those found in the Mount Jura block, and suggests that the sedimentary rock may have originally been under the volcanic rock, but has been faulted out. Similarly, eruptive volcanic rocks once overlying the sedimentary rocks of this sequence in the Mount Jura block may have been eroded or faulted away.
The Kettle Rock sequence is unconformably overlain by the Middle to Late Jurassic Mount Jura sequence. In the Mount Jura block, the Mount Jura sequence is composed of volcanic clast conglomerate, sandstone that is locally rich in shallow marine fossils including brachiopods and corals, and shale, which are overlain by feldspathic clastic rocks with thin tuff interbeds, and then overlain by pyroclastic rocks, ash flow tuff, and lesser shale, sandstone, and tuffaceous rocks. Fossils in the sequence in this block range from late Aalenian to Oxfordian (Harwood, 1992, Christe, 2020, 2021a), and detrital zircons from just above the base have an MDA of ~172 Ma (Aalenian; Christe, 2021a). A plagioclase porphyry andesite near the top of the sequence has a zircon U-Pb age of ~148 Ma (Tithonian age; Christe, 2020). In the other blocks, the Mount Jura sequence is made up of volcaniclastic rock with minor volcanic flow interbeds, intruded by a plagioclase-porphyry breccia and plagioclase-biotite-quartz porphyry sills and dikes. The plagioclase-porphyry breccia has a zircon U-Pb age of ~162 Ma, an ash flow tuff ~160 Ma, and an intrusive breccia in the tuff ~155 Ma (Christe, 2011, 2020). A granite dike in this sequence has yielded a muscovite Ar-Ar age of ~146 Ma (Dilles and Stephens, 2010).
The uppermost sequence of this terrane is the Evans Peak sequence. It is composed of chert pebble conglomerate, quartz sandstone, and shale/slate overlain by coarse-grained fossiliferous sandstone, shale, and volcanic-cobble conglomerate. The sequence grades from shallow marine to nonmarine. The fossiliferous sandstone of the upper part has many plant fossils and dinosaur-bone fragments. In the Mount Jura block, a tuff near the top of the fossiliferous section has yielded zircon U-Pb ages of ~127 and ~129 Ma, whereas in the Kettle Rock block a porphyry volcanic interbed near the base of the fossiliferous section has a zircon U-Pb age of ~128 Ma (all Barremian age; Christe, 2020). The base of the Evans Peak sequence is not well exposed, and where exposed a structural overprint is typical (Christe, 2020).
Unit is overthrust on the west by Paleozoic and Triassic units of the Northern Sierra terrane along the Taylorsville fault. The terrane is intruded by granitic plutons or stocks of the Diamond Mountains intrusive suite (unit Gdm) with Jurassic and Cretaceous U-Pb isotopic ages. Christe and Pecha (2012) report a diorite stock that intrudes across the bounding thrust with a zircon U-Pb age of ~117 Ma.