LATE NEOGENE AND QUATERNARY STRATIGRAPHY AND STRUCTURE OF LITTLE LAKE (WILLITS) VALLEY, NORTHERN COAST RANGE, CALIFORNIA

Little Lake Valley (town site of Willits) is an intermontane basin within the northern California Coast Ranges that contains a record of basin sedimentation and deformation during the Pleistocene. Little Lake Valley is remarkable because it is a basin containing a thick section of Quaternary to upper Neogene (?) sediment situated at a major drainage divide between the north-flowing Eel River and the south-flowing Russian River. Well logs indicate that Quaternary and upper Neogene (?) sediment fills the valley to depths greater than 140 m. On the basis of well logs as well as fault-line-scarp morphology, the eastern margin of the valley is bounded by an inactive steep fault, which has at least 50 m of displacement up to the east. In contrast, the actively creeping dextral Maacama fault trends north-northwest through the southwest corner of the valley. Although valley fill may be as old as Pliocene, tephra layers in the sediment fill at or near the surface have Pleistocene ages: Thermal Canyon tephra (740 ka), the Rockland tephra (~575 ka) and an unnamed tephra correlative to tephra in the Clear Lake basin (~110 ka). Valley fill mainly consists of fine grained lacustrine and overbank sediment and coarser grained gravel alluvium with minor amounts of colluvium on the east margin. Facies relationships inferred from well logs indicate that streams aggrading the valley were through-going streams traveling along the valley axis and not fan delta systems aggrading from steep valley sides. Valley fill strata tilt uniformly to the north with a mean dip of 8° and a range of 5° to 25°. Locally, sediments dip steeply in the Maacama fault zone. Paleo flow direction, as determined from clast imbrication and channel margin trends, is to the south; in contrast, surface drainage on the valley floor is presently to the north. We infer that the northward tilt of basin strata is part of a regional deformational response to northward passage of the Mendocino triple junction. A byproduct of the regional northward tilt is Little Lake Valley's reversal in drainage direction to the north in the late Pleistocene. We infer that basin northward tilt predates the northward propagation of the Maacama fault zone into the Little Lake Valley.