PALEOGENE SEDIMENTARY TECTONICS OF THE GREYBULL/BASIN AREA, NORTHWEST WYOMING: LITTLE DRY CREEK PARAFOLD AND GOULD BUTTE/ THREE SISTERS BLOCK

Field and satellite imagery studies of uppermost Cretaceous through lower Eocene sediments in an approximately 600 km² area west of the towns of Greybull and Basin, WY, and between Little Dry Creek (north) and Antelope Creek (south), demonstrate a complex history of syntectonic erosion and deposition marked by a series of stacked erosional and angular inter- and intraformational unconformities. Along Little Dry Creek, the systemic K-P boundary between the Lance Formation and the Fort Union Formation is a landmark angular unconformity, and two intraformational Fort Union angular unconformities are also identified by abrupt changes in strike and dip. The gently west-dipping lower Eocene Willwood Formation caps the southerly dipping uppermost Fort Union sequence. The Rio Thrust Fault exhibits local surface expression in a steep, west-southwesterly dipping parafold developed in lower and middle Fort Union rocks marked by sharp dip and strike changes along a probable tear fault. The Gould Butte/Three Sisters Block is a structurally and sedimentologically distinct terrain west of Greybull, WY that is bounded on the north by the blind, east-west trending Shell Lineament and on the south by a narrow zone of steeply dipping, normal-faulted Willwood sediments that trends N 40^o E. Seven unconformities in the Paleogene sequence mark punctuated episodes of deformation controlled by sporadic movement along the Rio Thrust Fault. Unlike most highly variegated lower Willwood sediments, the lower 200-300m of Willwood rocks on the Gould Butte/Three Sisters block are drab, indicating less mature paleosols and relatively more rapid and continuous sediment accumulation. South of the Gray Bull River, deformed Willwood strata are truncated by an erosion surface and covered by a major multistory-multilateral sandbody and avulsion deposits. Correlation of this unconformity with fossil mammal faunal turnover at Biohorizon A, and dating of Biohorizon A by its position relative to the Paleocene-Eocene boundary (56.33 MA) and the C24R to C24N geomagnetic reversal (53.57 Ma), indicate that the last of several punctuated movements on the middle segment of the Rio Thrust Fault took place between 55.24 and 55.06 Ma. Research funded by NGS Waitt grant W315-14 to AEC, NSF grants EAR-0739718 and EAR-0616430 to AEC, and NSF grant EAR-0616376 to KDR.