RYU, IN-CHANG, and A. R. NIEM, Department of Geosciences, Oregon State University, Corvallis, OR

ABSTRACT: Sequence Stratigraphy in Active Convergent Margins: Eocene Tyee Basin, Southern Oregon Coast Range

Eocene strata in the Tyee basin provide an excellent example of an ancient subduction zone complex and overlying forearc sequence. Rapid facies variations due to deposition on uneven basin topography related to developing tectonic structures, however, have led to difficulties in making lithostratigraphic correlations, which have resulted in conflicting depositional models. A 5-year sequence stratigraphic analysis reveals that several lithostratigraphic units, previously interpreted by others as related facies (e.g., Tyee Formation, delta-slope-fan), are reinterpreted as unrelated unconformity-bounded units.

Surface to subsurface bio- and lithostratigraphic correlation, using well-logs, foraminifers and coccoliths, seismic reflection profiles, and measured sections in conjunction with new geologic mapping by colleagues allows recognition of four depositional sequences (I to IV). Each sequence starts

with a lowstand systems tract (either as a prograding wedge and/or incised valley fill and/or slope-fan/basin-floor fan), overlain by a transgressive systems tract (backstepping parasequence sets with major marine flooding surface), and capped by a highstand systems tract (wave-dominated delta with shoreface progradational parasequence sets). Sequence boundaries are commonly characterized by local unconformities, rapid basinward shift in facies, and abrupt changes in stacking patterns. Lower Eocene depositional sequences I and II represent an asymmetrical trench fill or accretionary rifted margin complex (sediments derived from the Mesozoic Klamath Mountains) that was being actively subducted. These 12,000-ft thick sequences distally on lap a paleohigh of a subsiding oceanic island te rane to form a deep-marine condensed section <200 ft thick. Similarly, the overlying middle Eocene 8,000 ft thick forearc sequences III and IV appear to be controlled by irregular filling of lows and pinching out on highs on the underlying imbricate thrust fault topography. This study demonstrates that sequence stratigraphy is a useful tool for unraveling basin architecture along active margins.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.