INTERPRETATION OF A PRIMARY VOLCANIC FLOW DEPOSIT IN THE TUSCAN FORMATION, CHICO CALIFORNIA

The Tuscan Formation consists of a series of debris flow and fluvially reworked volcaniclastic deposits located in the Big Chico Creek Ecological Reserve in Northern California approximately 15 km east of Chico, northern California. Upper and lower age constraints are from the Ishi Tuff (1.8 Ma) and the Nomlaki Tuff (3.27 Ma), respectively. Within the Tuscan Formation, correlating between outcrops is difficult due to lateral facies variations and lack of dateable primary volcanic deposits. Recognition of any time correlative unit within the Tuscan Formation would add a third absolute age date as well as aid other on-going projects attempting to better characterize Tuscan stratigraphy and emplacement history. This study describes a potential primary volcanic flow deposit located 30 meters from the base of the Tuscan Formation, where it uncomformably overlies the lava flows of the Lovejoy Basalt. The flow unit of interest lies between lahar flow deposits of the Tuscan Formation. It is 1.5 meters thick and contains six distinguishable layers ranging from medium to fine grained ash to conglomerate beds with clasts up to 10 cm in diameter. The bottom layer is a 24 cm thick bed of medium to fine grained cross-bedded ash, which is overlain by a 19 cm thick bed of poorly sorted volcanic clasts in an ash matrix. Immediately above the poorly sorted unit is another 8 cm layer of medium to fine grained ash below two 24 cm beds of poorly sorted volcanic conglomerate, each with an ash matrix. The uppermost bed is 30 cm thick and also contains volcanic clasts in an ash matrix. Hand sample examination of matrix and clasts throughout the outcrop suggest the flow unit is monolithologic. Preliminary interpretations include the possibility that the outcrop is the deposit of a volcanic block and ash flow. If ongoing field work and further data support this interpretation, dating of the block and ash flow deposit would provide additional information to constrain the lower age of flow units of the Tuscan Formation.