AN EXAMPLE OF LATE EOCENE/EARLY OLIGOCENE SEA FLOOR TOPOGRAPHY IMAGED BY 3D SEISMIC DATA, MIST GAS FIELD, AND IN OUTCROP, N. COAST RANGE, OREGON

In the Mist Gas Field major unconformities are present on top of the middle to late Eocene Cowlitz, u. Eocene/l. Oligocene Keasey, and Sager Creek Fms. A 10 sq. km, high frequency, small bin (12 m) 3D seismic survey in the Mist Gas Field in Columbia Co., Oregon imaged the contact between the Keasey Fm. and the overlying Sager Creek Fm. The Keasey/Sager Creek surface resembles a deeply incised submarine channel system on the continental slope. The major channel in the 3D map area is 200 m deep and 1400 m wide. Smaller channels feed into the main channel. In the Mist Gas Field, the Keasey Fm. is composed of massive to laminated, deep marine (bathyal) tuffaceous clay rich siltst. with some glauconite ss stringers. The overlying channel-filling Sager Creek Fm. consists of deep marine (bathyal) thin, well bedded laminated micaceous ss, siltst. and mudst. In outcrop, w. of the gas field, nested channels of Sager Creek Fm. contain concretionary rhythmically thin bedded to laminated, v. f. gr. micaceous, carbonaceous lithic arkosic turbidite ss (Bouma cde sequences), and foram-bearing burrowed-laminated micaceous mudst. with flame and load structures. The 35 km long wedge-shaped turbidite unit (400 m thick) is channelized into the Keasey Fm. and may have been fed from the east by the Pebble Ck. delta front facies of the Pittsburg Bluff Fm. On well bore image logs the formational contact is sharp and distinct. Immediately above the contact a zone of mud-chip conglomerate or chaotic bedding of variable thickness (1m to 4 m) is overlain by laminated beds. Well bore radioactive logs suggest the mineralogy of the micaceous Sager Creek Fm. has an affinity to the older micaceous Cowlitz Fm. As the tuffaceous forearc strata of the Keasey Fm. were sourced from initial Western Cascade arc volcanism, the change in mineralogy may represent cessation of volcanism and reestablishment of regional drainage to the granitic/meta. source rocks east of the Cascades. L. Eocene normal faulting is the primary trapping mechanism in the Mist Gas Field. Seismic data clearly show that the trapping faults terminate before reaching the Keasey/Sager Creek surface. Reactivated post-m. Miocene conjugate oblique-slip faults have minor offsets in outcrop. The 3D imaged surface therefore is close in appearance to the original l. Eocene/e. Oligo. sea floor topography.