Beer Mug Anticline; Cored by Pennsylvanian Tensleep Ss
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Frontispiece. Aerial photograph with view to the northeast, emphasizing an asymmetric anticline (Beer Mug Anticline; cored by Pennsylvanian Tensleep Ss. and flanked by Permian Goose Egg Fm.) thrust atop a less obvious, synformal anticline (Ellis Ranch Anticline of Taylor, 1996; cored by Triassic Red Peak Fm. and stratigraphically overlain by the white, more clearly defined, Triassic Alcova Ls.). The steeply north-dipping thrust fault between the anticlines (Indian Spring Fault) is exposed only near the photo’s lower-left corner, and it strikes northeastward from there, hidden by the slopes of erosional debris. The camera is above the northern edge of the eastern Hanna Basin, and the general view is across southern parts of the adjacent Freezeout Hills. This image was selected for use as the Frontispiece jointly because of its clarity and for being emblematic of the extraordinary degree of Laramide structural complexity characteristic of the Hanna Basin area — and particularly so along the basin’s northern and southern margins. For structural details, see Taylor (1996, p. 31–50 and, espe- cially, fig. 7). Photograph courtesy of Dan Hayward (©Copyright 2011; HaywardPhoto.com, image taken July 30, 2011). Cover images: Top photo—View is to northwest, looking across southeastern parts of ‘The Breaks’ at the northeastern corner of Wyoming’s Hanna Basin. Shirley Mountains dominate the left skyline. Camera is placed at dead center of the W ½ of sec. 10, T. 23 N., R. 80 W., featuring lower Paleocene strata of the >4.8 km-thick Hanna Formation. Photographer stands on steeply southwest-dipping lower parts of hanging wall of the out-of-the-basin Dragonfly (thrust) Fault. As per usual for thrusts around the eastern half of the Hanna Basin, the Dragonfly Fault puts younger strata onto older, leading to structurally thinned sections around basin margins. On opposite side of the ephemeral drainage in the photo’s center is the top of the Dragonfly’s footwall. The steeply northeast-dipping rusty sandstone at the ridge’s top is the southwestern edge of the fault’s footwall syncline (the ‘Great Tortilla’ of Lillegraven et al., 2004, fig. 4B, RMG, v. 39, no. 1). Lower-left photo—Shown is a roughly 30 cm-wide band of tightly folded, alternating siltstone and coal found low in the hanging wall of the Dragonfly Fault (camera is directed northwest). This style of intense deformation is common at all scales adjacent to thrust faults around margins of the Hanna Basin, especially in hanging walls. Lower-right photo—Shown are prosaic yet essential tools that greatly aided the field research included in the following article. 30 Rocky Mountain Geology, v. 50, no. 1, July 2015 Downloaded from http://pubs.geoscienceworld.org/uwyo/rmg/article-pdf/50/1/30/3942910/gsrocky_50_1_30.pdf by guest on 23 September 2021 Late Laramide tectonic fragmentation of the eastern greater Green River Basin, Wyoming Jason A. Lillegraven Departments of Geology/Geophysics and Zoology/Physiology, Dept. 3006, 1000 E. University Ave., University of Wyoming, Laramie, Wyoming 82071-2000, U.S.A. email: [email protected] ABSTRACT KEY WORDS: allochthons, basement-involved thrusting, Carbon Today’s greater Green River Basin is limited to the southwestern quarter of Wyoming. Basin, Cretaceous, Eocene, greater From late in the Cretaceous into late Paleocene time, however, sedimentary accumulations Green River Basin, Hanna Basin, within that basin continued uninterruptedly much farther to the east, connecting areas klippen, Laramide Orogeny, out-of- now occupied by the isolated Hanna, Carbon, Pass Creek, and Laramie basins. Field-based the-basin thrust faults, Paleocene, research resulted in three contiguous geologic maps that focus on modern basin margins and paleogeography, Rocky Mountains, boundaries among those eastern elements. Analyses of derived cross sections and restored stratigraphy, tectonic evolution, stratigraphic columns suggest that active subsidence and rapid sedimentary accumulation Wyoming, Wyoming Basin, younger- persisted with only minor interruptions until very late in the Laramide Orogeny. That on-older thrust faults. led to a generally symmetrical north–south cross-sectional configuration of the original Hanna Basin, with its true depositional axis set well south of its apparent position of today. “Properly made geologic maps are the The Hanna Basin’s present strong asymmetry developed only secondarily. That basin’s most quantitative data in geoscience: modern configuration reflects Paleogene influences of: (1) late Laramide (early Eocene and While we may debate the nature of probably younger) basement-involved contractional tectonics and associated uplifts; (2) a contact, the contact and dip-strike out-of-the-basin thrusting passively responding to stratigraphic crowding; (3) prodigious measurements, if properly located, syntectonic erosion; and (4) resulting basin fragmentation. North–south dimension of the should be there 100–200 years hence late Paleocene (i.e., pre-fragmentation) greater Hanna Basin sedimentary sequence was and are therefore both quantitative and roughly twice that of today, and near-sea-level topographic conditions persisted until late reproducible, something that cannot be Eocene time. As expected, remnants of basin margins universally show major thinning of said of experiments in some of the other stratigraphic sequences. Principal thinning was from tectonic causes, however, exhibiting sciences.” erosional angular unconformities only rarely. Out-of-the-basin, younger-on-older faulting (in which fault planes cut down-section) accompanied by massive erosion was the rule at all A. M. Celâl Şengör (2014, p. 45) basin margins. Uplift of Simpson Ridge Anticline postdated deposition of upper Paleocene strata in direct continuity between what is now the separated Hanna and Carbon basins. INTRODUCTION The basement-involved fault system responsible for westward relative tectonic transport (and ca. 8 km of elevation) by Simpson Ridge also led to raising the attached Carbon Basin. Focus of Present Work Original Hanna Formation of the Carbon Basin was beveled away by erosion and soon thereafter became replaced by shallow-slope sliding of a long-runout allochthon, the Carbon What we refer to today as the Basin Klippe. The klippe’s original site of deposition probably was to the northeast, above Hanna and Carbon basins represent what later became Flat Top Anticline. Uplift of Flat Top and Simpson Ridge anticlines was two relatively small, eastern fragments essentially synchronous (latest Paleocene or, more probably, early Eocene), establishing a of an originally enormous, Cretaceous lengthy, faulted-synclinal separation of the Hanna/Carbon Basin from the Laramie Basin. and Paleocene greater Green River That syncline also bifurcated Simpson Ridge Anticline into western and eastern segments. Basin (Lillegraven et al., 2004, figs. A second allochthon, the Dana Klippe, rests upon southern parts of the Hanna Syncline 15 and 19). The primary objective of (of Hanna Basin). That klippe’s site of deposition probably was above the area that later this paper is to present hypotheses elevated as Elk Mountain, thus causing origin of Pass Creek Basin. Elk Mountain’s ca. 12 on how key steps in fragmentation km uplift could not have occurred prior to the early Eocene, and that event contributed to of the original eastern end of that tight folding of the Hanna Syncline. Coal Bank Basin is a giant footwall syncline ahead of greater Green River Basin came to be. the out-of-the-basin, thrust-faulted (with relative tectonic transport to the southwest) Dana Through the course of this research it Ridge Anticline. That fault–fold complex represents a common structural style seen at all was necessary to gather a great deal of scales across the Hanna Basin. primary stratigraphic, attitudinal, and Rocky Mountain Geology, June 2015, v. 50, no. 1, p. 30–118, doi: 10.2113/gsrocky.50.1.30, frontispiece, 12 figs., 2 tables, 2 appendices, 20 supplemental files 31 Received 3 March 2015 • Revised Submitted 6 July 2015 • Accepted 10 July 2015 • Published online July 2015 For permission to copy, contact [email protected] © 2015 University of Wyoming; Gold Open Access: This paper is published under the terms of the CC-BY license. Downloaded from http://pubs.geoscienceworld.org/uwyo/rmg/article-pdf/50/1/30/3942910/gsrocky_50_1_30.pdf by guest on 23 September 2021 Jason A. Lillegraven Contents FRONTISPIECE..................................................................30 Walcott Syncline, Coal Bank Basin, and Dana ABSTRACT ..........................................................................31 Ridge Anticline.............................................................74 KEY WORDS .......................................................................31 Dana Klippe..................................................................76 INTRODUCTION .............................................................31 Introduction to Dana Klippe...................................76 Focus of Present Work.................................................31 Basic Lithologic Generalizations of Dana Klippe........77 Essential Graphics and Appendices .........................35 Evidence in Support of an Allochthonous Origin....78 BACKGROUND RESEARCH FOR THIS INTRODUCTION TO THREE MODELED PROJECT...............................................................................37 EVOLUTIONARY SCENARIOS.....................................79 A CENTRAL CONCEPT FUNDAMENTAL TO THIS CONSIDERATIONS OF FIGURE 9 — SW–NE