The Tehama Formation at Black Butte Lake with a review of the geology of the Orland Buttes

Field surveys at Black Butte Lake in western Glenn and Tehama counties and a synthesis of the published literature of the Geology of the northern Sacramento Valley and have led to a revision of the stratigraphy and structure of the western side of the northern Sacramento Valley. The Orland Buttes are a series of flat topped mesas that bound Black Butte Lake on the east. The Buttes are composed of Upper Cretaceous marine rocks of the Great Valley Group, overlain by Oligocene to Miocene alluvium and capped by the middle Miocene (15.4Ma) Lovejoy Basalt. Alluvium of the Pliocene Tehama Formation and Pleistocene Riverbank Formation butt unconformably against the side of the Orland Buttes. Prior geologic maps show the Black Butte fault on the western slope of the Orland Buttes. This fault was proposed by Russell (1931) to explain the uplift of the buttes in relation to the surrounding Tehama Formation and was proposed as a source for the basalt that caps the Orland Buttes. The Basalt has subsequently been identified as a western exposure of the Lovejoy Basalt. The Orland Buttes are a series of Table Mountains similar to Oroville Table Mountain. There is no evidence to support the existence of the Black Butte fault. Oligocene to Miocene alluvium below the Lovejoy Basalt was not recognized in early surveys of the region. This alluvium is present on the north slope of North Butte where it was previously identified as the Tehama Formation. The alluvium was identified below the Lovejoy Basalt during construction of Black Butte Dam (US Army Corps of Engineers Foundation Report, 1963) and was informally named the Black Butte formation. Detailed descriptions of the lithology of the unit are available (US Army Corps of Engineers Foundation Report, 1963; Hancock et al., 1986). Lithologic descriptions compiled during field surveys reveal that the Black Butte formation is green to tan claystone interbedded with gravel and cobble conglomerate at the base of the unit. The fine-grained sediments are arkosic, the coarse-grained sediments contain clasts from the Sierra Nevada or Klamath Mountains. Previous researchers have correlated the Black Butte formation with other recognized units, but it is Oligocene to Miocene age and is a new lithologic unit. The Tehama Formation is an extensive deposit of Pliocene (3.27Ma) alluvium that is the primary aquifer on the west side of the Sacramento Valley. The Tehama Formation underlies both the Pleistocene Red Bluff Formation gravels and Riverbank Formation alluvium. North of Stony Creek the Tehama Formation is present in stream beds and below the elevation of the high water level along the shoreline of Black Butte Lake. South of Stony Creek the Tehama Formation is present in wave cut cliffs along the southern shoreline of the lake. On the eastern shoreline of the lake the Tehama Formation is present as perched terraces on the western slope of Middle Butte and North Butte. Detailed geologic mapping reveals that north and west of the Orland Buttes the surface exposures of the Tehama Formation are not as extensive as shown on prior geologic maps. Geologic maps produced by Russell (1931) depict exposures of the Tehama Formation north and west of the Orland Buttes, but he also explained in his PhD. dissertation that there was insufficient time to differentiate between the Tehama Formation and the overlying Red Bluff Formation in his maps of the region. This study verified that there are large areas west of the Orland Buttes that are covered by red iron oxide stained alluvium that are not shown on Russell’s geologic maps, or maps produced by subsequent researchers. This alluvium covers large parts of the study area and overlies the Tehama Formation along the lake shore and the Cretaceous rocks of the Great Valley Group west of the study area. No exposures of the Red Bluff Formation gravels were seen in the study area or west of The Orland Buttes. Two units of red, iron oxide stained alluvium are presently mapped in the study area. The Red Bluff Formation is a deposit of early Pleistocene sand and gravel which covers the Red Bluff Pediment. The Riverbank Formation is middle Pleistocene alluvium which occurs as terrace deposits along modern streams and alluvial fans on the floor of the Sacramento Valley. The red gravels north and west of the Orland Buttes are not a deposit of coarse grained alluvium of the Tehama Formation, nor are they Red Bluff Formation gravels. This study shows that the Iron-oxide stained alluvium north and west of the Orland Buttes is a fill terrace that was deposited into stream valleys that were eroded into the uplifted Red Bluff Pediment and the underlying Tehama Formation during the middle Pleistocene. This alluvium is middle Pleistocene, and should be re-assigned to the Riverbank Formation. These new interpretations of the stratigraphy in the study area will affect our understanding of the geology of the western side of the Sacramento Valley and the hydrologic properties of the Tehama Formation aquifer. This report demonstrates that large areas previously mapped as fine grained alluvium of the Tehama Formation actually consist of coarse grained sediments of the Pleistocene Riverbank Formation. The new interpretation of the geology and the differences in porosity and permeability of the rocks in the two geologic units may affect our understanding of the hydrologic properties of the aquifer. Many researchers extended faults into the study area as a result of Russel’s interpretation of the Black Butte fault. The Black Butte fault does not exist west of the Orland Buttes, the present interpretation of structural features in the northern Sacramento Valley will need to be re-evaluated.