Volcanic and sedimentary rocks
Volcanic and sedimentary rocks
Jurassic, Triassic, and Permian
This unit crops out in the northwestern part of Northern Sierra terrane. This unit is composed of three fault bounded blocks, a northern block where this unit overlies Paleozoic rocks questionably assigned to the Taylor Formation (part of the Taylorsville sequence in unit NSSB), and two southern blocks composed of Permian to Jurassic rocks of this unit.
The northern block (Butt Valley block of Jayko, 1990) is bounded on the west by the western block of this unit and on the east by rocks of the Shoo Fly Complex. In this block, the unit is composed of a basal conglomerate and sandstone overlain by a thin discontinuous limestone overlain by a thick sequence of volcaniclastic sandstone, argillite, and conglomerate with minor tuff and andesite sills/dikes. The basal conglomerate has yielded late Carnian to Norian conodonts (Jayko, 1988), the limestone (called Hosselkus Limestone by Jayko, 1990) has yielded Late Triassic (Norian) conodonts, and black slate in the upper unit has yielded probably Early Jurassic pelecypods (Jayko, 1990; Harwood, 1992), although Christe (2021b) states that this block includes marine strata as young as Late Jurassic (Oxfordian).
The western block (Soda Ravine block of Jayko, 1990) is bounded on the west by the Feather River peridotite belt, and south of the northern block on the east by rocks of the Shoo Fly Complex along with a lens of serpentinized peridotite. In this block, the unit is composed of slate, chert pebble conglomerate, and sandstone with intercalated limestone lenses. Limestone blocks crop out at two levels. The lower limestones have yielded Pennsylvanian to Early Permian conodonts and Early Permian fusulinids of McCloud affinity, whereas the upper limestones have yielded Middle to Late Triassic (Ladinian to Norian) conodonts, ammonites, and other fossils (Jayko, 1990; Harwood, 1992). Jayko (1990) calls the Norian limestones Hosselkus Limestone, as in the northern block. The sedimentary rocks structurally above the Paleozoic limestones are interlayered with quartz-, hornblende-, and plagioclase-porphyry flows, dikes and sills, along with a small stock (including the Humbug intrusives of Robinson, 1975). Jayko (1990) suggests that these are Triassic, because they are interleaved with the Hosselkus Limestone blocks, but Christe (2021b) describes zircon U-Pb ages spanning the late Aalenian through late Pliensbachian (Early to early Middle Jurassic), averaging ~178 Ma from the Humbug intrusives. As such, if the interleaved lava flows are related to the dated intrusives, the Hosselkus Limestone blocks are probably olistoliths in a Jurassic olistostrome. On the other hand, Christe (2021b) states that the Humbug intrusives lack the foliation of the surrounding metasedimentary rocks, so the metasedimentary and interleaved flow rocks could be older. The uppermost rocks in this block, east of the Triassic limestone, are a thick sequence of slate and phyllite with lesser sandstone and siltstone. These rocks must be Middle Jurassic or younger based on their stratigraphic position over the Humbug intrusives.
The southeastern block is a relatively narrow sliver bounded on all sides by Shoo Fly Complex. No fossils are reported from this sliver, but Harwood (1992) questionably correlates the rocks in this block with the western block based on the mapping of Standlee (1978).
Jayko (1990) builds on the McCloud affinity of the Paleozoic limestones to suggest that the rocks of the western block (and presumably the southeastern block) correlate with some, though not all, of the units in the Eastern Klamath terrane. Because the stratigraphic section is disrupted and incomplete, she interprets the block tentatively as an attenuated tectonic sliver of the Eastern Klamath terrane structurally emplaced along the west side of the Northern Sierra terrane. However, the Early to Middle Jurassic Humbug intrusives and overlying metasedimentary rocks are not like rocks in the Eastern Klamath terrane and preclude many of the correlations proposed by Jayko (1990). Mesozoic rocks in the northern block rest unconformably on Paleozoic rocks of the Northern Sierra terrane, but Jayko maps the Norian limestone as the same (Eastern Klamath) unit, Hosselkus Limestone, suggesting that the limestone was deposited across both Eastern Klamath and Northern Sierra terrane basement, or perhaps that the limestone bodies in this unit are olistoliths shed from the Eastern Klamath terrane onto basement of the Northern Sierra terrane. Christe (2021b) points out that rocks of this unit are not like coeval rocks in other parts of the Northern Sierra terrane (units NSTT and NSTS). We assign this unit to the Northern Sierra terrane largely based on the mapped depositional contact on rocks of another unit in the terrane. The differences are perhaps the result of differences in depositional and volcanic setting across the large extent of the terrane.