Digital-elevation models as reference maps for general orientation. A, State of Wyoming and parts of surrounding states, with orange rectangle indicating area presented in B. B, Most of southeastern Wyoming, locating main topographic features, rivers, and highways surrounding Hanna and Carbon basins; red polygons indicate boundaries of three geologic maps (Figs. 4–6; combined in Fig. 12) that serve as focus for this study. More detailed digital-elevation models in Figure 3 provide additional geographic reference and transect locations for geologic cross sections.
Stratigraphic sequence as recognized within Figures 4–12, including formational/group names, their geochronologic designations, maximum thicknesses observed within mapped boundaries (note qualification, below, for Ferris Fm.), and stratigraphic symbols/colors used. Undulating lines represent erosionally unconformable stratigraphic contacts. Sidebar column provides relative thicknesses of individual rock units* comprising this over 14 km- (roughly 9 mi-) thick Hanna/Carbon Basin composite sequence. Note overwhelming dominance of strata deposited during Late Cretaceous into early Eocene time, capped by two geographically separated, late Laramide, gravity-emplaced klippen from outside immediate study area. Lowry et al. (1973, sheet 1), considering basins under discussion here, simplified discussion of groundwater (its occurrence, quality, potential for development, and relation to surface water) in context of geology by dividing the local stratigraphic column into eight hydrographic ‘Units.’ Applied to presently used mapping (exclusive of Unit 8), the equivalents are:
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*Qualification on thickness of Ferris Formation — The roughly 2.5 km (1.6 mi) thickness shown here is a typical observable value within the mapped area, which reflects the usual condition of consequential loss of section along the basin margins due to effects of younger-on-older thrust faulting. But in the southern Hanna Basin, on both flanks of the Hanna Syncline (see Figs. 6, 7 [cross section F–G], and 8 [column F–G]), the relatively unfaulted Ferris Formation (there uniquely measuring from central parts of the basin) measures closer to 5 km (3.1 mi) in thickness.
Digital-elevation models of eastern half of Hanna Basin, Carbon Basin, and adjacent areas. A, Indicated are names of U.S. Geological Survey 7.5-minute topographic quadrangle maps (used as bases for geologic mapping throughout this study) and outlines of boundaries for three geologic maps (Figs. 4–6, combined in Fig. 12). Small inset ‘Breaks Map’ is part of Figure 5. B, Reference locations for interpretive geological cross sections plotted as map transects on Figures 4–6 and fully rendered in Figure 7. For convenience in use, all information presented in Parts A and B also is duplicated on each geologic map (Figs. 4–6) along with with cross sections (Fig. 7) and derived stratigraphic columns (Fig. 8). Principal contents of Figures 4–8 all employ 1:24,000 scale. Figures 9–11, developed using 1:100,000 scale, employ different placements of cross-sectional transects and numbering systems (see diversity of inset maps).
Geologic map of northeastern margins of Hanna Basin as seen south of Shirley Mountains and southwestern corner of Freezeout Hills (refer to Fig. 1B). Colored map at full 1:24,000 scale is presented in digital form suitable for printing as ‘Fig. 4.PDF’. A thumbnail version for quick reference, however, exists unscaled on following page. Indicated geological cross sections are presented in Figure 7. Unless otherwise indicated, strike/dip measurements are by the author, and associated station numbers (e.g., ‘2049’) allow access to additional geographic and geologic information listed under those numbers in Appendices 1 and 2 (Table 2). Exceptions on this map include attitudinal measurements identified as ‘LVI’ (from Van Ingen, 1978) and ‘DAT’ (from Taylor, 1996). In sections 23–25 of T. 24 N., R. 81 W. are plotted transects of ‘Legs 18, 19, and 20’ of a measured section that were not included in figure 4 as presented by Lillegraven et al. (2004).
Geologic map of entire eastern margin of Hanna Basin and all but southern extremes of Carbon Basin. Map is bounded to east by western parts of Flat Top and Big Medicine Bow anticlines, to north by course of Medicine Bow River, and to south by U.S. Interstate Highway 80 at southern extreme of Simpson Ridge Anticline. Colored map at full 1:24,000 scale is presented in digital form suitable for printing as ‘Fig. 5.PDF’. A thumbnail version for quick reference, however, exists unscaled on following page. Indicated geological cross sections are presented using 1:24,000 scale in Figure 7 (note that transects D–L and J–K continue westward onto Fig. 6). Unless otherwise indicated, strike/dip measurements are by the author, and associated station numbers (e.g., ‘5745’) allow access to additional geographic and geologic information listed under those numbers in Appendices 1 and 2 (Table 2). Exceptions on this map include attitudinal measurements shown: (1) without station numbers in the map's northeastern corner (from Blevens, 1984); (2) as ‘Taylor’ in northern part of inset (from Taylor, 1996); (3) with initials ‘DBH’ (from Dobbin et al., 1929) in western parts of Como West quadrangle; and (4) without station numbers on core and eastern flanks of Simpson Ridge Anticline (from Kraatz, 2002). Inset of ‘The Breaks’ at northern edge of map is rescaled (original 1:12,000) from figure 4B of Lillegraven et al. (2004), with cross section P′–Q′ re-labelled here from the original as A–A′.
Geologic map of southern parts of Hanna Basin. Map is approximately bounded to north by U.S. Highway 30/287, to south by northern topographic base of Elk Mountain, to west by northern end of Dana (Pass Creek) Ridge, and to east by western edge of southern half of Figure 5. Colored map at 1:24,000 scale is presented in digital form suitable for printing as ‘Fig. 6.PDF’. A thumbnail version for quick reference, however, exists unscaled on following page. Indicated geological cross sections are presented in Figure 7 (note that transects D–L and J–K continue onto Fig. 5). Unless otherwise indicated, strike/dip measurements are by the author, and associated station numbers (e.g., ‘7812’) allow access to additional geographic and geologic information listed under those numbers in Appendices 1 and 2 (Table 2). Exceptions on this map include attitudinal measurements shown as: (1) ‘BH’ (from Hitchens, 1999) on Dana Ridge and Coal Bank Basin; and (2) ‘RHB’ (from Beckwith, 1941) along southern edges of map. Dotted, east–west straight-line contacts between underlying strata and Hanna Formation of Dana Klippe on its northern edge and along northeastern flanks of Dana Ridge reflect prohibition of author's access by landowner for purposes of this research.
Geologic cross sections following transects shown in Figures 4–6. Illustrated is surface topography (scale 1:24,000 with no vertical exaggeration), with geologic features interpreted consistently to depth of 4,500 feet (1,372 m) above mean sea level. Sheet of colored sections at 1:24,000 scale is presented in digital form suitable for printing as ‘Fig. 7.PDF’. A thumbnail version for quick reference, however, exists unscaled on following page. Dips of strata, or their apparent dips (‘ap.’) as appropriate, are specified in degrees from horizontal, and each is: (1) graphically represented by a short black line projecting downward from surface at the relevant angle from its point of measurement; and (2) identified by a black station number (e.g., 6052). Additional geographic and lithologic information is provided for rocks at each point of measurement in Appendices 1 and 2, readily accessed by sequentially ordered station number (Table 2). Quadrangle, township, and section boundaries are indicated along baselines of all transects. Vertical, thin red lines serve to subdivide each cross section into natural units by which cumulative thicknesses can be measured along surface transects with progressively varying dips. Between each pair of red vertical lines are sets of red numbers, which represent individual components of measured thicknesses. Measurements are numbered sequentially and ordered stratigraphically up-section (see corresponding numbers on matching stratigraphic columns in Fig. 8). Thickness (in meters) of each individual stratigraphic unit was digitally measured (from screen-enlarged cross sections) normal to field-recognizable features such as dips of strata, fault planes, or formational contacts. Cross section F–G (which traverses Elk Mountain) is unique within this paper in being only in part restricted to landscape mapped here (attitudinal measurements identified as ‘RHB’ are from Beckwith, 1941). Cross section P–Q is duplicated (and reduced in scale from 1:12,000 to 1:24,000) from figure 4C in Lillegraven et al. (2004), where it was identified as cross section A–A. Cross sections R–S and T–U are duplicated from Clemens and Lillegraven, 2013 (there as fig. 5). Notice that almost all faulting on this sheet is interpreted to have been in the nature of out-of-the-basin, décollement-type thrust faults. [See ‘Qualification on thickness of Ferris Formation’ in caption to Fig. 2.]
Geologic columns (36 total, each column with two representations—one above and another below the red dotted, mostly horizontal line), constructed from field data presented in cross sections of Figure 7. Sheet of colored columns at 1:24,000 scale is presented in digital form suitable for printing as ‘Fig. 8.PDF’. A thumbnail version for quick reference, however, exists unscaled on following page. Individual columns are variously correlated stratigraphically by use of four, broadly relevant markers (specifically: approximate base of Lewis Sh., using maps in this paper; Ferris–Medicine Bow formational contact, using maps in this paper; approximate boundary of Torrejonian–Tiffanian land-mammal ages, of Higgins, 2003b; and occurrence of ‘78 coal,’ as mapped by Dobbin et al., 1929). Columns above red dotted line are presented as they would be measured during field encounters today (i.e., faults and unconformities might be recognized on outcrops, but they would not affect reporting of measurable thicknesses). Columns below red dotted line, in contrast, provide reasonable estimates (i.e., conservatively expressed interpretations based upon geographically nearby, more complete sections) of reductions in thickness of stratified rocks (indicated by narrow blue vertical lines) resulting from a combination of Laramide faulting and erosion (see Figs. 9–11; such gaps are not considered here in strata older than Steele Sh.). Stratigraphically ordered red numbers (to left of columns) represent corresponding, individually measured increments of rock as shown and explained in Figure 7, with their total thicknesses (in meters). Purpose of this figure is to graphically provide comparisons between estimates of original depositional thicknesses (upper columns) and today's thicknesses following faulting and erosion (lower columns). [See ‘Qualification on thickness of Ferris Formation’ in caption to Fig. 2.]
Interpretive cross-sectional model of late Laramide tectonic and erosional evolution across eastern Hanna Basin, following transect 1–2–3 on geologic reference map (which conjoins Figs. 4–6). A sheet of evolutionary cross sections and reference map at scale of 1:100,000 is presented in digital form suitable for printing as ‘Fig. 9 (trans 1–3).PDF’. A thumbnail version for quick reference, however, exists unscaled on following page. Cross section A postdates early deformation of Ferris Formation and deposition of Hanna Formation but predates principal deformation of the latter. An earlier stage would show deposition of Hanna Formation extended to southwest of marker-point ‘1’ into Saratoga Basin, where any correlative remnants today, if they existed, would be designated as Coalmont Formation (such designation by Montagne, 1991, pls. I and III is questionable — see text). Solid colors in A indicate strata remaining today, whereas partially transparent colors represent strata lost through erosion or faulting prior to completion of B. According to this interpretation, as suggested by Lillegraven et al. (2004, fig. 15), deposition of Hanna Formation originally overtopped and buried the Freezeout Hills (see northeastern end of cross section A). Cross section B depicts configuration of today's Hanna Basin. Also notice in cross sections A and B that, contrary to palynostratigraphic expectation, Paleocene pollen assemblage ‘P5’ sensu Nichols and Ott (1978, 2006) and Nichols (2009) underlies pollen assemblage ‘P3’ (see Lillegraven and McKenna, 2008) and mammalian assemblages (Higgins, 2003b) of Torrejonian (early Paleocene) age. Representation of basement origin for out-of-the-basin thrust faulting across the gap between Saint Marys Hill and Dana Ridge anticlines seems clear (see Mount et al., 2011, fig. 5), but the actual situation this far south remains quite unknown. [See ‘Qualification on thickness of Ferris Formation’ in caption to Fig. 2.]
Interpretive cross-sectional model of late Laramide origin of Carbon Basin Klippe, following transect 4–5–6 on geologic reference map. A sheet of evolutionary cross sections and reference map at scale of 1:100,000 is presented in digital form suitable for printing as ‘Fig. 10 (trans 4–6).PDF’. A thumbnail version for quick reference, however, exists unscaled on following page. Note 77˚ change in vector of transect at reference point 5 (in southwestern part of Carbon Basin). Solid colors in cross section A indicate strata remaining today in section D, whereas partially transparent colors represent strata lost through erosion or faulting prior to completed development of section D. Cross section A is intended to represent initial conditions during Paleocene time, through which most of Hanna Formation had been deposited. A normal basinal east–west stratigraphic sequence then ran generally uninterrupted (contra Ryan, 1977) across a flat landscape that later became deformed into Simpson Ridge Anticline. A progressively thinning section presumably ran eastward into what is now Laramie Basin. Cross section B indicates origin of a basement-involved thrust complex having relative tectonic transport to west that led to early uplift of Simpson Ridge Anticline (see Kraatz, 2002), eventually resulting in physical separation of Hanna Basin from Carbon Basin. Basement rocks below developing Simpson Ridge Anticline were elevated by about six kilometers. That led both to incipient stages of out-of-the-basin thrusting against steepening anticlinal flanks along southeastern margin of Hanna Basin and to erosion of originally present Hanna Formation from markedly uplifting area of newly developing Carbon Basin. Cross section C, in addition to continuing definition of Simpson Ridge Anticline, reflects origin of genuinely spectacular anticlinal folding of landscape (‘area of northeast-trending folds’; Fig. 1B) between Carbon Basin and Laramie Mountains (see Blackstone, 1983 and 1993a plus Lillegraven and Snoke, 1996). These were post-Paleocene events, because lower Eocene strata in Hanna Basin were profoundly deformed, both as related to uplift of Simpson Ridge and to northern elements in area of northeast-trending folds. This also would have been probable time of dumptruck-style, southwest-directed, overland sliding of Carbon Basin Klippe (composed of Hanna Fm. as deposited in originally more expansive formation within area that is now atop northern parts of Flat Top Anticline) along an irregular detachment surface to its present abutment against eastern flanks of Simpson Ridge. Thick red line (shown crossing multiple stratigraphic units) indicates then-future position of today's land surface. Cross section D shows fully modern development of Simpson Ridge Anticline, eastern Hanna Basin, Carbon Basin, and Carbon Basin Klippe. Configuration of basement-involved faulting within Simpson Ridge Anticline is simplified here from Kraatz (2002, figs. 12–15) for diagrammatic purposes.
Interpretive cross-sectional model of late Laramide tectonic and erosional evolution of Elk Mountain and Dana Klippe, following transect 7–8–9 on geologic reference map. A sheet of evolutionary cross sections and reference map at scale of 1:100,000 is presented in digital form suitable for printing as ‘Fig. 11 (trans 7–9).PDF’. A thumbnail version for quick reference, however, exists unscaled on following page. Cross sections A–C (which contain reference points 7–9) derive from sections H–I combined with F–G of Figure 7. Reference point 8 represents a spatial jump between two discrete places on laterally correlative strata along crest of Halleck Ridge (which is the northwestern limb of Bloody Lake Anticline). A 1:24,000-scaled cross section of existing geologic relationships (components from Fig. 7, secs. H–I + F–G) is at top right of figure. Cross section C is part of that same (i.e., present-day) transect but interpreted to greater crustal depth. Solid colors in cross sections A and B indicate rocks remaining today, whereas partially transparent colors represent rocks lost through erosion or faulting prior to completed development of cross section C. Cross section A is intended to represent initial conditions, in which Medicine Bow Mountains (well south of transect shown) already had commenced uplift and Hanna Formation was deposited but had not yet undergone consequential deformation. Presumptive positions of then-future thrusts are indicated. Thin dashed lines (to left) in Steele Shale and Mesaverde Group–Ferris Formation indicate approximate then-future positions of vertical left edge of cross section C following deep-centered fault motions that led to rotations (indicated by broad yellow arrows). Yellow star indicates approximate then-future position of eastern summit of Elk Mountain. Thick red line indicates then-future approximate stratigraphic position of (today's) land surface following emplacement of Dana Klippe. Cross section B is a hypothesized intermediate stage in which basement-involved thrusting having relative tectonic transport to the southeast had initiated early uplift of Elk Mountain (thus separating what is now Pass Creek Basin from an originally contiguous and southerly more-extensive Hanna Basin) in addition to out-of-the-basin thrusting along Elk Mountain's northern and southern flanks. Also indicated is earliest initiation (within strata directly above then-future Elk Mountain) of gravitational effects upon lower components of Hanna Formation to form Dana Klippe. This step predated its miles-long northwesterly slide across a shallowly dipping detachment surface to its modern position atop Hanna Syncline in southern Hanna Basin. Two thick red lines (shown crossing multiple stratigraphic units) indicate then-future position of today's land surface following emplacement of Dana Klippe. Cross section C requires at least five kilometers vertical uplift of basement rocks that form Elk Mountain and floor of Pass Creek Basin above conditions seen in cross section 11B (and at least 12 km above condition seen in cross section 11A). Lower right-hand digital-elevation reference map illustrates distribution of remaining strata mapped as Hanna Formation on high flanks of Medicine Bow Mountains.
Table 1.Principal data-bearing graphical elements and appendices underpinning this publication.
Notes: The scales designated below for Figures 4–11 apply to the full digital versions available for download and printing via the indicated URLs (duplicated following ‘REFERENCES CITED’). The ‘thumbnail’ versions, Figures 1–12, interspersed within this paper's text are intended for online reference only, and they can be viewed at any desired scale.
Figure 1. Digital-elevation models as reference maps for general geographical orientation.
Figure 2. Stratigraphic sequences, abbreviations, and colors as used throughout this paper.
Figure 3. Digital elevation models of study area indicating topographic quadrangle names, geologic map boundaries, and alignments of cross sections (the ‘Breaks Map’ in part A is a reduced version of fig. 4B from Lillegraven et al., 2004).