Abstract

The Hanna Basin of south-central Wyoming has been considered anomalous among other Laramide depocenters of the Rocky Mountain region because of its combination of small size and great thickness of synorogenic strata. Most prior interpretations of the Hanna Basin have assumed a history of subsidence and sedimentary infilling discrete from surrounding basins. In contrast, we summarize new geological and paleontological information from the northeastern corner of the modern Hanna Basin suggesting that, prior to late Paleocene time, the Hanna Basin and nearby Carbon, Pass Creek, Laramie, and Shirley Basins were unified and depositionally continuous with the much larger Green River Basin to the west. Only late in the local expression of the Laramide orogeny (late Paleocene and early Eocene) did this “greater Green River Basin” become subdivided through completion of development of intrabasinal, basement-involved thrust faulting and associated anticlines. We view the present Hanna Basin as only a small, structurally defined remnant of an enormous, ponded basin that extended eastward during most of Paleocene time from the Wyoming–Idaho–Utah thrust belt to the newly uplifted Laramie Mountains. That basin was bounded on the south by the Uinta Mountains and combined Sierra Madre–Medicine Bow Mountains and on the north by the Gros Ventre Range, Wind River Mountains, and Sweetwater arch. When the original configuration of the unified greater Green River Basin is taken into account and combined with palinspastic removal of late Laramide faulting that defines its various margins, this basin at least rivaled dimensions of the Powder River Basin of Wyoming and Montana; the greater Green River Basin greatly exceeded the volume of sedimentary accumulation within the Powder River Basin. Rates of Paleogene basinal erosion associated with structural subdivision of eastern components of the greater Green River Basin were prodigious.

Within context of a measured section, we describe and interpret the lithologic nature, depositional settings, source areas, previous geographic extent, and deformational history of the Hanna Formation as seen today in the eastern Hanna Basin. Temporal control is linked to summaries of included mammalian fossils, freshwater molluscs, leaf macrofloras, and palynomorphic assemblages. Unexpectedly early occurrences of certain palynomorphic assemblages suggest that paleoecological controls on distributions of pollen-forming plants during Paleocene time were particularly important.

Deformation of strata in the eastern Hanna Basin chiefly reflects a complex interaction between a major, south-directed, low-angle, basement-involved thrust system and the thick and generally incompetent, mudstonerich Hanna Formation. We argue that out-of-the-basin faults, the result of spatial crowding in the basin, are kinematically linked to the basement-involved thrust system through a triangle-zone geometry. This trishear-style of deformation involved north-directed, out-of-the-basin, décollement-type thrusts and a synchronous south-directed, basement-involved, blind-thrust system. Map relationships along the northern margin of the basin indicate that many of the out-of-the-basin faults emplaced younger rocks onto older rocks, seemingly an anomalous structural relationship for contractional deformation. One explanation of these relationships involves synchrony of the: (1) cutting down section of north-directed, out-of-the-basin fault planes; with (2) upturning of basinal strata related to south-directed, basement-involved faulting along the northern margin of the Hanna Basin.

Finally, the inherited Precambrian structural grain of the Archean Wyoming province was instrumental in controlling the orientation of many of the Laramide-age, basement-involved structural features. Tectonic heredity, rather than changing stress orientations, probably accounts for the variation in orientations of structural features associated with the northern and eastern margins of the Hanna Basin.

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