Pennsylvanian sinistral faults along the southwest boundary of the Uncompahgre uplift, Ancestral Rocky ,

William A. Thomas* Department of Geological Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA

ABSTRACT sylvanian-Permian Ancestral interpretations, and the conclusions provide have attracted a wide range of interpretations, testable implications for integration of regional Resolution of the large-scale kinematics of each of which ultimately depends on the time and kinematics. the Pennsylvanian-Permian Ancestral Rocky sense of slip on each of the specifi c faults within The Pennsylvanian-Permian Ancestral Rocky Mountains requires defi nition of sense of a large array of generally high-relief basement Mountains structures refl ect protracted tectonic slip and time of movement on specifi c faults blocks (Fig. 1) (e.g., Baars, 1966; Kluth and inheritance from Precambrian basement faults, within the system of fault-bounded basins Coney, 1981; Baars and Stevenson, 1982, 1984; as well as a history of episodic reactivation of and uplifts (e.g., Paradox basin, Uncompah- Kluth, 1986, 1998; Stevenson and Baars, 1986; Precambrian and Paleozoic faults (e.g., Baars gre uplift). Along an east-west–striking seg- Thomas and Baars, 1995; Ye et al., 1996; Mar- and See, 1968). In turn, the Ancestral Rocky ment of the boundary between the Paradox shak et al., 2000; Dickinson and Lawton, 2003). Mountains structures have been variously basin and Uncompahgre uplift, a system of Thick accumulations of locally derived, coarse overprinted by Cretaceous-Eocene Laramide steep faults defi nes a and sys- clastic sediment of Pennsylvanian-Permian age basement-rooted structures, and the relative tem, which includes the Grenadier fault provide the primary evidence for the large-scale magnitudes of Ancestral Rockies and Laramide block. Positive fl ower structures () structure of the Ancestral Rocky Mountains. components of deformation remain in question along both the bounding (Coal Bank Pass Fault boundaries between large-scale basement along many structures. This article begins with a and Molas Creek faults) and internal (Snow- uplifts (e.g., Uncompahgre and ) summary of pre-Pennsylvanian fault movements don fault) faults of the Grenadier fault block and basins (e.g., Paradox and ) and concludes with an evaluation of Pennsylva- indicate strike-slip displacement, and oblique are mapped from scattered exposures of faults, nian-age fault movements in the regional con- subsidiary folds show sinistral slip. On both sedimentary thickness and facies distribution, text of the Ancestral Rocky Mountains and pos- sides of the fault, stratigraphic and subsurface data (e.g., summaries in Baars, sible Laramide overprints. thinning of parts of the Pennsylvanian Her- 1966; Mallory, 1972; Rascoe and Baars, 1972; Parts of the boundary-fault system between mosa Group toward the fault documents Weimer, 1980; Baars et al., 1988). For many of the Uncompahgre uplift and the Paradox basin synsedimentary growth of the positive fl ower the faults, the dip is not well constrained, and a on the southwest are exposed in the Laramide- structure, suggesting Pennsylvanian sinistral clear defi nition of slip sense is rare. age San Juan in southwestern Colorado slip along this part of the Ancestral Rocky Resolution of the various large-scale tectonic (Fig. 1). In that area, a system of faults defi nes Mountains. Sinistral strike-slip along one models for the Ancestral Rocky Mountains has blocks (the Grenadier and east-west–striking segment of the fault sys- been hampered by lack of data for the time and Sneffels fault blocks, Fig. 1) within the north- tem between the Paradox basin and Uncom- sense of slip on specifi c faults from which inter- eastern part of the Paradox basin (Baars, 1966; pahgre uplift is compatible with reverse slip pretations of an integrated regional fi eld Baars and See, 1968; Weimer, 1980). A Penn- on northwest-striking segments, demon- can be derived. This article reports on faults sylvanian-Permian (Hermosa Group and Cutler strating that a regional assembly of data to along the Grenadier fault block, which is within Group) succession fi lls the Paradox basin, over- defi ne sense of slip and time of movement on a system of faults at the boundary between the laps the Grenadier and Sneffels fault blocks, specifi c faults will better constrain regional- southwestern side of the Uncompahgre uplift and pinches out against the Ridgeway fault at scale models for kinematics and mechanics of and the Paradox basin in southwestern Colorado, the southwest boundary of the Uncompahgre Ancestral Rocky Mountains deformation. where the time and sense of slip can be inter- uplift. The Ridgeway fault strikes east-west for preted. The objective is similar to that of a study some distance, constituting a “kink” in the gen- Keywords: Ancestral Rocky Mountains, sinis- of faults along the boundary between the north- erally northwest-striking boundary between the tral fault, synsedimentary fault, Pennsylvanian, eastern side of the Uncompahgre uplift and the Uncompahgre uplift and Paradox basin (Fig. 1) Uncompahgre uplift. Central Colorado trough in southern Colorado, (Stevenson and Baars, 1986), and the bound- where sedimentary facies and folded angular aries of both the Sneffels and Grenadier fault INTRODUCTION unconformities document Pennsylvanian-Perm- blocks strike approximately east-west, parallel ian synsedimentary northeast-directed thrusting with the Ridgeway fault. West of the kink, the The large-scale tectonic setting and driving (Hoy and Ridgway, 2002). While these exam- boundary between the Uncompahgre uplift and mechanisms of the basement faults of the Penn- ples alone will not resolve the regional-scale Paradox basin curves to northwest strike. East of mechanics, they illustrate the kinds of data that the kink, the Sneffels and Grenadier fault blocks *[email protected]. are necessary for resolution of the alternative along with the uplift boundary bend abruptly to

Geosphere; June 2007; v. 3; no. 3; p. 119–132; doi: 10.1130/GES00068.1; 10 fi gures.

For permission to copy, contact [email protected] 119 © 2007 Geological Society of America

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Front Utah Central Colorado tro Colorado Range Uncompahgre

Paradox

basin u UNCOMP gh

Figure 1. Index map of Ances- AHGRE UPLIFT tral Rocky Mountains, show- Gateway, CO ing locations of faults along the boundary between the Uncom- Ridgeway pahgre uplift and the Paradox fault basin (compiled from Larsen Area of and Cross, 1956; Baars and See, Figure 2 1968; Weimer, 1980; Frahme and Vaughn, 1983; Hoy and PA Ridgway, 2002). Dashed-line RADO Snef SC fels block -C circle shows location of the Laramide San Juan dome. SC- X BASI Gre C—Sand Creek–Crestone fault. nadier block Utah Colorado N New Mexico

N

100 km os-Picuris fault

Pec 36°N 110°W

the southeast and south (Fig. 1). These struc- sedimentary cover (Figs. 1 and 2)1. The Coal granite, indicating a nonconformable sedi- tures are now exposed in Precambrian to Penn- Bank Pass and Molas Creek faults are part of a mentary cover over crystalline basement rocks sylvanian rocks in the structurally highest part roughly en echelon fault set, which includes the (Barker, 1969; Gibson, 1990); alternatively, a of the Laramide San Juan dome. The Laramide Snowdon, Andrews Lake, and Little Molas Lake zone of sheared, foliated rocks along the con- structure is framed by relatively low-angle dips faults within the Grenadier fault block (Fig. 2). tact suggests a thrust boundary (Tewksbury, in Permian and younger rocks around the west Excellent exposure permits detailed observation 1985). Cuspate infolding of the cover strata and south sides of the dome; however, Tertiary of structure and stratigraphy in the area of the into the basement provides a mechanism for rocks of the San Juan volcanic fi eld overlap Pre- east-striking fault system, supporting interpreta- thrusting along the contact between the par- cambrian rocks on the north and east sides of the tions of time and sense of fault slip. autochthonous Uncompahgre Group and the core of the dome. basement rocks (Harris et al., 1987, Fig. 3 Precambrian therein). A convex-upward geometry of the PRE-PENNSYLVANIAN TECTONIC limbs of cuspate folds is expressed in upwardly HISTORY OF THE GRENADIER FAULT The Precambrian Uncompahgre Group con- and outwardly decreasing dips, and the limbs BLOCK sists of alternate units of quartzite and phyl- are broken by strike-slip zones. Although lite, each with a range of thickness averaging a cuspate-infold geometry adequately explains The Grenadier fault block consists of Pre- ~200 m. The Uncompahgre Group physically the east-striking elongate outcrop area of the cambrian quartzite and phyllite (Uncompahgre overlies higher grade Precambrian gneiss and Uncompahgre Group between contacts with the Group) with a Paleozoic sedimentary cover. The older Precambrian gneiss and granite, the con- bounding faults of the Grenadier fault block, tact subsequently was displaced by steep faults the Coal Bank Pass and Molas Creek faults on 1If you are viewing the PDF of this paper, or if with substantial vertical separation, defi ning you are reading this offl ine, please visit http://dx.doi. the south and north, respectively, juxtapose the org/10.1130/GES00068.S1 or the full-text article on a structurally low block (Grenadier graben) Uncompahgre Group and cover against older www.gsajournals.org to access the layered PDF of between the Coal Bank Pass and Molas Creek Precambrian gneiss and granite with a Paleozoic Figure 2. faults (Baars and See, 1968).

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0 sections at X end points of the cross trace of Cambrian-Mississippian rocks; ) extend along the outcrop 310 ′ outline of Figure 4 outline of Figure A X , Y–Y ′ 42'30" (X–X 5 (A–A sections in Figure of the map. Map shows locations lines cross area shown 7A, 7B, 9A, 9B, 10A, and 10B are shown by outlines. Locations of photographs in Figures 6 are aerial photograph in Figure contour lines are projected from measured bedding attitudes throughout the map area. Layer 3—Lines of cross sections in Figures 3—Lines of cross Layer the map area. bedding attitudes throughout measured from projected lines are contour Figure 2 (see footnote 1). Geologic map of the Grenadier fault block between the Coal Bank Pass and Molas Creek faults (base ma fault block between the Coal Bank Pass and Molas Creek 1). Geologic map of the Grenadier 2 (see footnote Figure The map is compiled interval 40 ft = 12.2 m). values in feet; topographic contour Peak 7.5-min quadrangles; topographic contour map of base Pennsylvanian strata (base Molas Formation) (str contour 2—Structure 1—Geologic map. Layer combination. Layer contour structure for elevation control of base Molas Formation provide m). Elevations of outcrops interval 100 contour ture

Geosphere, June 2007 121

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Cambrian ite boulders indicate a steep scarp between the North of the Molas Creek fault, Ignacio eroded surface on the Uncompahgre Group quartzite-boulder conglomerate rests on older The Upper Cambrian Ignacio Formation north of the Coal Bank Pass fault and that on the Precambrian granite, and the formation thickens is characterized by quartzose sandstone and Twilight Gneiss south of the fault. and grades abruptly to sandstone northward away locally includes quartzite-boulder (50+ cm) Quartzite-boulder conglomerate is scattered from the fault (sections 17–21, Y–Y′ in Fig. 3) conglomerate in irregular elongate areas along along the south side of the Snowdon fault, where (Baars and See, 1968). The Molas Creek fault the Coal Bank Pass, Snowdon, and Molas Creek the conglomerate rests on quartzite and phyllite is a zone that includes two distinct faults. The faults (Baars and See, 1968; Block, 1986). within the Uncompahgre Group (Fig. 2; section southern branch of the Molas Creek fault zone The rounded quartzite boulders are lithologi- 15, X–X′ in Fig. 3). On the fault block (south- is an east-striking fault that defi nes the northern cally identical to quartzite in the Uncompahgre ern part of Grenadier fault block) between the boundary of the Grenadier fault block and sepa- Group, indicating local sources from Uncom- Snowdon and Coal Bank Pass faults, the Ignacio rates the Uncompahgre quartzite on the south pahgre quartzite. boulder conglomerate thins abruptly southward from Precambrian granite on the north. Devo- The Ignacio quartzite-clast conglomerate and away from the Snowdon fault, and the forma- nian-Mississippian strata overlap the southern sandstone south of the Coal Bank Pass fault rest tion either pinches out or is represented only branch of the Molas Creek fault zone, indicat- nonconformably on the older Precambrian Twi- by a pavement conglomerate (a single bed with ing no post-Devonian fault movement (Y–Y′ in light Gneiss, and the entire Ignacio Formation thickness equal to the diameter of single clasts) Fig. 3). The northern branch of the Molas Creek pinches out northward a few meters south of the over the southern part of the block (sections 8– fault zone diverges to the east-northeast with fault (sections 5–7, X–X′ in Fig. 3). The boulder- 14 in contrast to 15, X–X′ in Fig. 3). North of the Precambrian granite on both sides, but the fault conglomerate beds pinch out southward away Snowdon fault, on the fault block (northern part displaces beds as young as Pennsylvanian, dis- from the Coal Bank Pass fault into a southward of Grenadier fault block) between the Snowdon tinguishing the post-Devonian northern branch thickening sandstone succession (from section and Molas Creek faults, the Ignacio Formation from the pre-Devonian southern branch (Fig. 2). 7 to section 3, X–X′ in Fig. 3). Distribution of is unconformably absent, and Devonian strata Ignacio quartzite-boulder conglomerate overlies the quartzite boulders indicates supply from overlie the Uncompahgre Group quartzite (sec- the Precambrian granite along the north side Uncompahgre Group quartzite in the Grenadier tion 16, Y–Y′ in Fig. 3). The restriction of the of the southern branch and extends northward fault block north of the Coal Bank Pass fault and Ignacio quartzite-boulder conglomerate to the across the northern branch of the Molas Creek proximal transport southward onto paleotopo- south side of the Snowdon fault indicates a fault zone. Devonian strata unconformably over- graphically lower Twilight Gneiss south of the south-facing paleotopographic scarp, along the lie Uncompahgre quartzite south of the southern fault (Baars and See, 1968; Block, 1986). The base of which boulders were concentrated on branch of the Molas Creek fault zone on the large size and local distribution of the quartz- the Uncompahgre bedrock. Grenadier fault block. Distribution of the Ignacio

Coal Bank Snowdon X X′ MOLAS FORMATION South Pass fault fault North 123 4 56789 10 1112 13 14 15 LEADVILLE LIMESTONE AND OURAY LIMESTONE dolostone limestone

ELBERT FORMATION

MCCRACKEN SANDSTONE MEMBER Y Molas Creek Y′ IGNACIO FORMATION South fault North 16 17 18 19 20 21 boulder conglomerate

50 m UNCOMPAHGRE GROUP quartzite Scale phyllite 400 m PRECAMBRIAN gneiss, schist, granite

Figure 3. Stratigraphic cross sections showing thickness and facies distributions of Cambrian through Mississippian formations with respect to locations of faults and Precambrian rock types. Datum is base of Pennsylvanian Molas Formation. The lines of cross section extend along the outcrop trace of Cambrian-Mississippian rocks, partly within the area of Figure 2, which shows the locations of end points of the cross sections. The line of cross section X–X′ is offset at the Coal Bank Pass fault, and the line of cross section Y–Y′ is offset at the northern branch of the Molas Creek fault zone. Sources of measured stratigraphic sections: 1, 5, 13, 16, 18, and 21 from Baars and See (1968); 1–7 and 18–20 from Block (1986); 6–15, 17, and 18 from fi eld measurements by author.

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boulder conglomerate shows that boulders from lithology and nearly uniform in thickness across Evans, 2002). The coarsening-upward clastic paleotopographically high Uncompahgre quartz- both the Coal Bank Pass and Molas Creek faults; successions are generally no more than 20 m ite were transported onto paleotopographically however, it thins locally over small fault blocks thick, suggesting the maximum paleobathym- lower Precambrian granite and were concen- north of the Coal Bank Pass fault (sections 8–9, etry of the delta front. The coarse clastic sedi- trated along a north-facing scarp at the southern X–X′ in Fig. 3) and is very thin or absent locally ments indicate proximity to a sediment source branch of the Molas Creek fault zone. along the south side of the Snowdon fault in the general area of the Uncompahgre uplift. Distribution of Ignacio quartzite-boulder con- (Fig. 2). The distribution of the upper part of the Extensive shallow-marine limestone units inter- glomerate along the faults indicates synsedimen- Elbert Formation indicates subdued paleotopog- bedded with the deltaic successions are consis- tary south-facing scarps along the Coal Bank raphy across both boundaries of the Grenadier tent with cyclic eustatic changes in sea level, Pass and Snowdon faults and a north-facing scarp fault block and possibly minor fault movement which are well documented by evaporite cycles along the southern branch of the Molas Creek within the block. in the more distal, central part of the Paradox fault zone. The juxtaposition of the Uncompah- The regionally extensive Devonian Ouray basin (e.g., Peterson, 1966; Hite and Cater, gre Group against older Precambrian igneous Limestone and Mississippian Leadville Lime- 1972). The shallow-marine to deltaic facies and metamorphic rocks on the north and south stone both are represented in a generally dolo- are sensitive indicators of minor differences in indicates a structurally low Grenadier fault block mitized and relatively massive carbonate unit on paleotopographic/paleobathymetric relief and, in Precambrian time. Paleotopographic scarps, and adjacent to the Grenadier fault block (Baars thus, are useful in recognition of synsedimen- facing away from the Grenadier fault block dur- and See, 1968). A karst surface on the carbonate tary structural relief. Facies variations in the ing deposition of the Ignacio quartzite-boulder unit is overlain unconformably by red beds of lower part of the Hermosa Group are inter- conglomerate, may have been a result of differen- the Pennsylvanian Molas Formation. The thick- preted to indicate paleobathymetric relief at the tial erosion of the structurally higher fault blocks ness of preserved carbonate beneath the uncon- boundaries of the Grenadier fault block (Spoel- on both north and south (Baars and See, 1968) formity varies across and adjacent to the Coal hof, 1976). Local truncation of Paleozoic units and/or of a component of fault after Bank Pass, Snowdon, and Molas Creek faults beneath the basal Hermosa beds on the south pre-Ignacio erosional planation. In contrast, both (Fig. 3), suggesting fault reactivation along the side of the Snowdon fault, as well as quartzite sides of the Snowdon fault are in the Uncom- Grenadier fault block. Locally, nondolomitized boulders (from the Uncompahgre Group) in pahgre Group. A south-facing paleotopographic limestone in the uppermost preserved beds con- the lowermost Hermosa Group just north of the scarp followed the trace of the Snowdon fault tains Devonian fossils, indicating local tilting fault, indicates relative uplift of the south side of rather than Uncompahgre stratigraphic units, and erosional truncation of the Mississippian the Snowdon fault (Spoelhof, 1976). indicating that the scarp refl ects fault movement, Leadville part of the carbonate succession prior The Hermosa Group is overlain by the either pre- or syndepositional with respect to the to deposition of the Molas Formation (sections coarser red-bed succession of the Permian Cut- Ignacio boulder conglomerate. 13–14, X–X′ in Fig. 3) (Baars and See, 1968). ler Group (e.g., Baars, 1966; Baars et al., 1988), Locally along the south side of the Snowdon which refl ects an increase in clastic sediment Devonian-Mississippian fault, the Devonian-Mississippian units are thin supply from the Uncompahgre uplift. Trunca- to absent (Spoelhof, 1976). tion of folded Cutler Group beds at an angular The McCracken Sandstone Member of the unconformity beneath Triassic strata documents Upper Devonian Elbert Formation unconform- PENNSYLVANIAN-PERMIAN Permian deformation along the Sneffels fault ably overlies the Ignacio Formation, thins STRATIGRAPHY block (Weimer, 1980). northward toward the Coal Bank Pass fault, and pinches out northward approximately at The red beds (dominantly mudstone and the fault (section 7, X–X′ in Fig. 3) (Baars siltstone) of the Pennsylvanian Molas Forma- and See, 1968). Between the Coal Bank Pass tion are highly variable in thickness on a local Fault Block between the Snowdon and Coal and Molas Creek faults, the McCracken Sand- scale, refl ecting paleotopographic relief on the Bank Pass Faults stone Member is generally absent (Fig. 3) but karst surface on the Ouray-Leadville carbonate may be represented by sporadically distributed rocks (Baars and See, 1968; Spoelhof, 1976). The fault block between the Snowdon and sandstone along the south side of the Snowdon The fi ne-grained red beds of the Molas Forma- Coal Bank Pass faults consists of nearly verti- fault (Fig. 2). In an approximate mirror image tion grade up into a succession of gray shale, cal units of quartzite and phyllite in the Uncom- of the block south of the Coal Bank Pass fault, sandstone, and limestone of the Pennsylvanian pahgre Group folded by a vertically plunging, the McCracken Sandstone Member thins south- Hermosa Group. north-facing, map-scale (Figs. 2 and 4) ward toward the Molas Creek fault (sections The Pennsylvanian Hermosa Group is a thick (Harris et al., 1987). On the northwest-striking 17–21, Y–Y′ in Fig. 3) and is absent directly (~870 m), cyclic succession of shallow-marine western limb, the vertical quartzite-phyllite suc- south of the southern branch of the fault zone limestone, mudstone, and deltaic sandstone cession is unconformably overlain by an Elbert- (section 16, Y–Y′ in Fig. 3). The distribution of units (e.g., Baars, 1966; Spoelhof, 1976; Evans, Ouray-Leadville succession with an average the McCracken Sandstone Member duplicates 2002). The limestone units are typically shal- angular discordance of ~90° (X–X′ in Fig. 3). that of the Ignacio Formation, indicating that the lowing-upward successions no more than 12 m The Elbert-Ouray-Leadville succession is Late Cambrian paleotopography was replicated thick (Spoelhof, 1976). Cyclic clastic succes- folded into fl at-bottomed and north- in the Late Devonian, either by differential ero- sions of mudstones, containing marine fossils; east-verging asymmetric anticlines broken by sion or by minor fault reactivation. sandstones, containing fragmented plant fossils; some steep faults (Fig. 4). Exemplifying the The Elbert Formation of shale and thin-bed- and coarser grained, cross-stratifi ed sandstones geometry of the folds, the Ouray-Leadville ded carbonate rocks above the McCracken Sand- with scoured bases are interpreted to be prodelta, carbonate can be traced along both limbs and stone Member varies little across the area (Baars distal-bar to delta-front, distributary-channel, around the plunging nose of the most south- and See, 1968). The succession is uniform in and delta-plain deposits (e.g., Spoelhof, 1976; erly (Fig. 4). The angle of dip on the

Geosphere, June 2007 123

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35 71 A' Sno 78 wdon 44 33 69 fault 10 82 5 3 12 6 6 82 88 63 16 12 B' 7 88 7 12 8 C' 12 A 10 15 19 10 13 9 27 25 44 25 87 80 25 45 16 9 19 15 11 80 15 12 B 16 6

20

C 5

Coal Bank Pass fault

N 0 500 m Pennsylvanian Hermosa Group 0 1000 ft A A′ 10,000 Pennsylvanian Molas Formation

feet Mississippian Leadville Limestone and 9500 Devonian Ouray Limestone undivided B′ Devonian Elbert Formation and B Cambrian Ignacio Formation undivided 10,000 Precambrian Uncompahgre Group

feet quartzite C 9500 phyllite

C′ Precambrian Twilight Gneiss

10,000 feet 9500

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Figure 4. Detailed map and structural cross sections of western part of fault block between the Coal Bank Pass and Snowdon faults, show- ing relation of folds in Paleozoic cover strata to contacts between quartzite and phyllite units in the unconformably underlying Precambrian Uncompahgre Group (base map from Engineer Mountain 7.5-min quadrangle; topographic contour values in feet; topographic contour interval 40 ft = 12.2 m). Location of the map is shown by outline in Figure 2. Bedding-attitude symbols show data synthesized from >400 measurements, which were used in construction of the map and cross sections. Vertical scale of cross sections with no vertical exaggeration is shown in feet for comparison with topographic contour map. In the cross sections, the red lines show faults, and the blue lines above the topographic profi le show the contacts of Paleozoic stratigraphic units projected up plunge into the plane of each cross section. In the undivided Ignacio-Elbert map unit, a mappable thickness of Ignacio is limited to the area south of the Coal Bank Pass fault and to a narrow area adjacent to the south side of the Snowdon fault (see Fig. 3 and discussion in text).

steep northeast limb increases southeastward between the vertically dipping quartzite and Pennsylvanian Hermosa Group on the north, the along strike and up plunge, commensurate with phyllite units (Fig. 4) indicates a genetic associa- maximum vertical separation on the Snowdon increase in amplitude of the conical . A tion of differential upward displacement of the fault is ~900 m (Fig. 2; E–E′ in Fig. 5). The steep fault breaks the up-plunge part of the steep phyllite with the anticlinal folding of the cover down-to-north vertical separation indicates limb. The steep limb and the fault in the cover succession. The systematic asymmetry of the inversion of the sense of separation indicated strata are aligned above the contact between the anticlines is consistent with compressional, top- by the distribution of the Cambrian Ignacio underlying vertical phyllite and quartzite, and the to-northeast displacement of the phyllite with quartzite-boulder conglomerate along the Snow- fault displaces the phyllite upward into the core respect to the quartzite. The geometry of the don fault. A straight map trace across the canyon of the anticline in the cover strata with respect folds documents northeast-southwest shortening of Lime Creek shows that the fault is nearly ver- to the quartzite beneath the adjacent syncline parallel with bedding in the Elbert-Ouray-Lead- tical (Fig. 2). Along the Snowdon fault, a narrow in the cover strata. Farther north, bedding atti- ville succession and perpendicular to in array of anastomosing faults bounds lozenge- tudes and outcrop patterns indicate the limbs of the phyllite. The folds in the cover succession shaped horses of various Pennsylvanian and two other, more deeply eroded anticlines. Some may have been a passive response to deforma- older stratigraphic units, and other faults splay steep faults with small displacement appear to tion of the phyllite; however, bedding-parallel at small angles from the primary fault (Fig. 2). be nonsystematically distributed (Fig. 4). shortening of the cover succession is consistent Westward along strike within <3.5 km from the The folds and fold hinges in the Paleozoic with compression perpendicular to foliation in point of maximum separation, the vertical sepa- strata are systematically distributed with respect the phyllite. No data are available to discrimi- ration decreases to zero; and a distinct to phyllite and quartzite units in the unconform- nate between distributed slip on phyllite foliation ends westward in the core of a steep, symmetric, ably underlying Uncompahgre Group. Anticlines or brittle faulting; however, the thickness of the upright anticline in the Hermosa Group (Fig. 2; in the cover strata are positioned over vertical phyllite units must have been reduced commen- B–B′ in Fig. 5). Small faults splay into the steep phyllite units, and fl at-bottomed synclines in the surate with the shortening in the cover succession limbs of the anticline. cover strata are positioned over vertical quartzite across each anticline. Asymmetry of the folds is In the fault block north of the Snowdon units. Fold hinges in the Elbert-Ouray-Leadville consistent with northeast-southwest compres- fault, the Hermosa beds are folded in a distinct succession are positioned above and are parallel sion, and the en echelon arrangement of the fold anticline-syncline pair (Figs. 2 and 6; D–D′ with the strike of the lithologic contacts in the noses indicates sinistral transpression between in Fig. 5). The fold axes trend approximately underlying, vertical succession of quartzite and the Snowdon and Coal Bank Pass faults. N70W and intersect the generally west-striking phyllite units (Fig. 4). Wavelengths of the folds The time of folding in the block between Snowdon fault at angles of 20° to 30° (Fig. 2). in the Paleozoic strata correspond to the strati- the Snowdon and Coal Bank Pass faults is The folds plunge away from the fault, and graphic thickness of the phyllite and quartzite not constrained by available data. The Elbert- amplitude decreases away from the fault down units, averaging ~200 m. An average enveloping Ouray-Leadville succession thickens gradually the plunge of the folds. surface defi ned on the unconformably truncated northward across the fault block but shows no Determining the sense of slip on the Snow- quartzite units is relatively planar, but the phyl- systematic relationship to the distribution of don fault is critical to regional interpretation of lite extends above the average truncated-quartz- quartzite and phyllite in the substrate (Fig. 3). kinematics of the Ancestral Rocky Mountain ite surface into the cores of the anticlines in the The folds are clearly expressed in the Ouray- structures. The steep dip and down-to-north cover strata (Fig. 4). The structural relief of the Leadville massive carbonate; however, the pres- vertical separation are compatible with dip- folded unconformity corresponds to that of the ent outcrop trace of the base of the unconform- slip displacement. In excellent exposures along overlying Paleozoic strata, even though bed- ably overlying Molas Formation is northwest of Lime Creek canyon (readily viewed from an ding in the underlying metasedimentary rocks is the northwest ends of the plunging folds (Fig. 4), overlook on U.S. Highway 550, Fig. 7), north- approximately perpendicular to the palinspastic precluding determination of whether the folds dipping Hermosa beds on the north side of the (prefolding) confi guration of the unconformity. are younger or older than Molas deposition. Snowdon fault have the appearance of a drag Each of three anticlines plunges and fl attens fold, juxtaposed against upthrown Uncompah- northeastward along strike of the underlying Snowdon Fault gre metasedimentary rocks, indicating down-to- quartzite-phyllite, and the plunging noses of the north dip slip. The north-dipping beds, however, anticlines defi ne a sinistral, en echelon align- The Snowdon fault is within the Grenadier are the south limb of the syncline that diverges ment, which trends northward between the Coal fault block of the Precambrian Uncompah- obliquely from the Snowdon fault, and that fold Bank Pass and Snowdon faults (Fig. 4). gre Group and Paleozoic cover strata (Fig. 2). limb continues westward as the north limb of The alignment of hinges in the folded Elbert- Locally, where the Uncompahgre Group on the the symmetric anticline into which the Snowdon Ouray-Leadville succession with contacts south is juxtaposed against the upper part of the fault ends westward along strike. Furthermore,

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/3/119/865160/i1553-040X-3-3-119.pdf by guest on 26 September 2021 Thomas 11 10 Lake fault 8 Lake fault Little Molas Little Molas Coal Bank Pass, Snow- 7 48 15 fault fault fault 11 6 7 5 Molas Creek Molas Creek Molas Creek J 2 7 Andrews Andrews Lake fault I Andrews Andrews 2500 . 3000 Lake fault ′ Group north of the Snowdon fault (Fig. Group 8) H 3500 m 3000 2500 3500 m 18 3000 2500 t, straight lines with numbers show apparent dips t, straight lines with numbers show apparent 10 3500 m 27 12 Figure 8 57 22 fault fault 1000 m 38 Snowdon Snowdon Snowdon Snowdon G F 3500 2500 2000 3000 2000 3000 2500 4000 m 3500 m surface

olas Formation

M

f present topographic

base o Andrews Andrews Lake fault B’ 4 8 23 41 33 51 29 35 41 25 17 46 15 Figure 8 27 37 fault fault 42 41 fault fault 23 Snowdon Snowdon Snowdon Snowdon 15 Snowdon Snowdon Snowdon Snowdon 41 2 9 Figure 9 3 C DD’ E A’ 12 2000 3000 2500 (base of Molas Formation) showing along-strike variations in vertical separation along the les of base Pennsylvanian rocks 19 3500 m ; angular unconformity within the Hermosa Group south of the Snowdon fault (Fig. 9) is in cross section B–B south of the Snowdon fault (Fig. 9) is in cross unconformity within the Hermosa Group ; angular 3000 2500 2000 ′ 3000 2500 2000 44 22 3500 m 41 3500 m 41 25 16 Pass fault Coal Bank Coal Bank Pass fault Coal Bank 3000 2500 3000 2500 3500 m 3500 m don, and Molas Creek faults. Lines of cross section are shown in Figure The scale of the vertical axes shows elevation. Shor shown in Figure 2. section are faults. Lines of cross don, and Molas Creek Southward thinning of part the Hermosa bedding-attitude measurements. from section computed and projected in plane of cross section F–F is in cross Figure 5. Structural profi Structural 5. Figure

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Andrews Lake fault Figure 6. Aerial photograph of topographic fl atirons on folded limestone and sandstone beds H-5 of Hermosa Group north of the Snowdon fault, showing trends H-4 H-6 of fold axes and locations of tra- verses of measured sections (in anticline Fig. 8). Location of the aerial H-3 photograph is shown by outline in Figure 2. H-2 N sync line H-1

Snowdon fault 500 m

Figure 7. Photographs of north-dipping Hermosa beds north of the Snowdon fault. Orientations of views are shown in Figure 2. (A) Distant view of folded Hermosa beds north (left in view) of the Snowdon fault; the Hermosa beds are juxtaposed against Uncompahgre Group quartzite and phyllite along the Snowdon fault (view to east). Within the Hermosa beds, a syncline-anticline pair diverges from the Snow- don fault and plunges northwestward (toward the lower left corner of the photograph). The Snowdon fault extends obliquely across the view from just south (right in view) of the highway in the lower right corner of the photograph, and passes between Snowdon Peak (on the horizon) and the folded Hermosa beds. (B) Closer view of synclinally folded Hermosa beds north (left in view) of the Snowdon fault, which is out of view off the right side of the photograph (view to east). The north-dipping Hermosa beds have the appearance of a drag fold on a down-to-north normal fault; however, the north-dipping beds are the common limb of a syncline that diverges westward from the Snowdon fault and the anticline into which the Snowdon fault ends westward (Fig. 2). In the anticline west of this view, Hermosa beds south of the Snowdon fault dip southward away from the fault (view in Fig. 9A), showing similar dips in both directions away from the fault.

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west of Lime Creek canyon, Hermosa beds in the symmetric anticline and the small splay faults enabling measurement of several stratigraphic the symmetric anticline dip both north and south characterize a positive fl ower structure (Hard- sections and precise correlation by outcrop away from the Snowdon fault, indicating that ing, 1985). Subhorizontal slickenlines observed tracing between them. Five stratigraphic sec- these dipping beds are not dip-slip drag folds. locally along the fault support an interpretation tions of the middle part of the Hermosa Group Alternatively, the abrupt along-strike decrease of strike-slip movement. The divergence of the were measured, using conventional fi eld tech- in separation, the along-strike termination of the northwesterly plunging fold axes from the west- niques, at sites spread along an alignment away fault in an upright anticline, the small lozenge- striking Snowdon fault indicates sinistral strike- from the Snowdon fault (Figs. 6 and 8). The shaped horses, and the geometry of the fault slip displacement. sections include multiple, well-exposed units splays, along with the steep dip of the primary Stratigraphic relationships can be used to of limestone and sandstone, as well as inter- fault, conform to the characteristics of a strike- constrain the time of the fault movement. On the vening shale units that are poorly exposed. In slip fault (e.g., Wilcox et al., 1973; Sylvester, northern fault block, the Hermosa stratigraphy addition, correlations were extended to a pre- 1988). Termination of the fault along strike into is excellently exposed in large fl atirons (Fig. 6), viously published measured section (Spoelhof, 1976) farther from the Snowdon fault (Figs. 6 and 8). The measured sections farthest north of South North the Snowdon fault document a relatively uni- H-1 H-2 H-3H -4 H-5 form thickness of the stratigraphic succession, including specifi c sandstone and limestone H-6 units (sections H-4 to H-6, Fig. 8). In con- trast, the measured sections closer to the fault show progressive thinning toward the fault of the cumulative succession and of specifi c units H-1 within it (sections H-4 to H-1, Fig. 8). Gradual EXPLANATION thinning is evident within 400 m of the fault, H-2 and thinning is most pronounced within 200 m Limestone 50 m of the fault (Fig. 8). Synsedimentary dip angles Sandstone may be calculated from the thickness gradients. A calculation of incremental synsedimentary Shale and cover dips, within stratigraphic intervals ~70 m thick, 150 m shows dips as steep as 18° northward within Snowdon fault 200 m of the fault. The cumulative dip angles in the lower part of a thicker stratigraphic inter- H-3 val are steeper (Fig. 8), indicating progressive rotation of the beds upward toward the fault. Synsedimentary dip angles decrease northward away from the fault. Lack of complete exposure precludes a defi nitive determination of whether the thinning is a result of angular unconformi- H-4 H-6 ties or of progressive thinning of successive H-5 units; however, stratigraphic thinning toward the Snowdon fault is clearly documented South North (Fig. 8). Thinning toward the fault indicates a H-1 H-2 H-3 H-4 H-5 synsedimentary growing uplift with northward H-6 dip away from the north side of the Snowdon fault during Hermosa deposition. On much of the structurally higher block south of the Snowdon fault, the Hermosa Group has been eroded, and Hermosa beds are preserved only to the west down plunge, including in the Snowdon fault 150 m anticline near the western end of the fault (Fig. 2). A high cut along U.S. Highway 550 exposes the steep south limb of the symmetric positive fl ower 150 m structure (anticline) and an abrupt hinge to more Figure 8. Stratigraphic cross section of beds in Hermosa Group north of the Snowdon fault. gentle dips (Figs. 2 and 9A; B–B′ in Fig. 5). Locations of traverses of measured sections are shown in Figure 6. Because of outcrop ori- Within the gently dipping Hermosa beds south entations, the traverses of the measured sections are oblique to the trace of the Snowdon of the hinge, an angular unconformity between fault. The plane of this cross section is perpendicular to the Snowdon fault; the traverses a sandstone unit and underlying beds of shale of the measured sections are projected onto the plane of the cross section to show thickness and sandstone has an angular discordance of ~4° variations with respect to distance from the fault. Cross section with 3× vertical exaggera- and truncates >8 m of beds (Fig. 9). The beds tion illustrates correlations between measured sections, and cross section with no vertical below the angular unconformity dip south- exaggeration shows true angular relationships. Section H-6 is from Spoelhof (1976). ward, the same direction as the south limb

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Figure 9. Photographs of angular unconformity in Hermosa beds exposed along U.S. Highway 550 south of the Snowdon fault. Orientations of the views are shown in Figure 2, and the location is shown in Figure 5. (A) Steep south limb of anticline at west end of the Snowdon fault; the angular unconformity is exposed beneath more gently dipping beds south (left in view) of the abrupt hinge on the south limb of the anti- cline (view to west). The crest of the anticline and the trace of the Snowdon fault are out of the view to the north (right in view). (B) Angular unconformity exposed in highway cut (view to north). The hinge and steep up-turn of the south limb of the anticline are hidden behind the shoulder of the highway cut. The Snowdon fault crosses the highway approximately at the position of the most distant car on the highway. The north-dipping beds in the distance are in the north limb of the anticline on the north side of the Snowdon fault.

of the anticline. The exposure of the angular thinning toward the Snowdon fault within the the Snowdon fault could be Laramide, that can- unconformity extends southward from 200 m to Hermosa Group are less than the present struc- not be confi rmed because of lack of preserved 400 m away from the Snowdon fault. The angu- tural dips on both limbs of the positive fl ower post-Pennsylvanian strata over the fault. lar unconformity exposed in the highway cut and structure (anticline) associated with the Snow- a similar unconformity stratigraphically higher don fault (Fig. 5), and the beds that show thin- Coal Bank Pass Fault in the Hermosa Group on the same fold limb ning toward the fault were subsequently folded. have been interpreted to indicate synsedimentary The structural similarities suggest a progression The Coal Bank Pass fault is the boundary movement on the Snowdon fault and associated from a low-amplitude synsedimentary anticline between the Uncompahgre Group on the Grena- folds (Baars, 1966; Baars and See, 1968; Spoel- into the present positive fl ower structure along dier fault block and the Precambrian Twilight hof, 1976). Alternatively, the angular discor- the Snowdon fault in a persistent kinematic sys- Gneiss on the south (Fig. 2). The Coal Bank Pass dances have been interpreted, in the context of tem. By analogy with the coincident later struc- fault is geometrically similar to the Snowdon deltaic depositional environments of the Hermosa ture, the synsedimentary Hermosa structure is fault within the Grenadier fault block, including Group, to be a result of downlap of delta-front interpreted to be a sinistral strike-slip fault and steep dip and anastomosing splays. Maximum foresets and truncation beneath reworked delta- positive fl ower structure. vertical separation of the base of the Paleozoic platform deposits (Evans, 2002). The positions The location of the Snowdon fault was inher- cover is at least 200 m, but separation decreases of these unique structures adjacent to the trace of ited from the fault that controlled deposition of northwestward along strike (Fig. 2; A–A′ and the Snowdon fault and the associated anticline, the Cambrian Ignacio boulder conglomerate. B–B′ in Fig. 5). The Coal Bank Pass fault ends as well as the consistency of dip directions of the The age of the latest movements on the Snow- northwestward in the core of an upright, steep, truncated beds and of the anticline limb, suggest don fault and the associated positive fl ower symmetric anticline in the Hermosa Group. The a genetic relationship to synsedimentary faulting structure are not specifi cally constrained; how- fault in the Hermosa Group was inherited from and folding. Synsedimentary slump faults dis- ever, both structures end westward within strata the boundary fault between the Uncompahgre place beds within the steep limb of the anticline of the Hermosa Group down plunge and down Group and the Twilight Gneiss. The geometry (Spoelhof, 1976; Evans, 2002). the dip on the west fl ank of the Laramide San of the fault and associated anticline in Hermosa The southward dip of beds beneath the angu- Juan dome. The Hermosa Group dips westward beds conforms to that of a positive fl ower struc- lar unconformity south of the Snowdon fault is beneath the Permian Cutler Group west of the ture along a strike-slip fault. Northwest of the similar in style and magnitude to the dip calcu- western end of the Snowdon fault, and whether end of the Coal Bank Pass fault, and northwest lated from northward thickening in the succes- the fault might have projected upward into the of the northwest-plunging anticline, the Her- sion north of the fault. The opposite directions Cutler Group cannot be determined. All of the mosa beds dip westward beneath the Permian of synsedimentary dip indicate an upright anti- structural relief of the Snowdon fault could be Cutler Group, which apparently is not deformed cline with a crest along the Snowdon fault. The Pennsylvanian age, and no beds younger than by the fault or the fold. Hermosa synsedimentary anticline is identical Hermosa can be shown to be displaced by the in location and geometry, but not in amplitude, Snowdon fault. Laramide structure as expressed Andrews Lake and Little Molas Lake Faults to the present structure of the Snowdon fault and in post-Pennsylvanian rocks consists of rela- positive fl ower structure (anticline). The incre- tively gentle westward dip from the crest of the The Andrews Lake fault and Little Molas Lake mental dip angles indicated by stratigraphic San Juan dome. Although some reactivation of fault are exposed entirely within the Hermosa

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Group (Figs. 2 and 5). Maximum vertical sepa- bed within a succession of sandstone and shale decreases westward along strike, and the fault ration is small (<50 m for Andrews Lake fault, units is broken by sandstone dikes (Fig. 10B). ends westward in an asymmetric anticline in the <10 m for Little Molas Lake fault), and both fault Some dikes appear randomly oriented; however, lower part of the Hermosa Group (Fig. 2; J–J′ to traces end along strike in both directions. Because most of the sandstone dikes are arrayed in G–G′ in Fig. 5). The termination of the Molas of the small vertical separation on these faults, any distinct hexagonal patterns. The hexagonal Creek fault in an anticline in Hermosa beds, as variations in dip away from or toward the faults sets are interconnected and aligned along well as the anastomosing branches along the are too small to be recognized. The Little Molas a N75W trend adjacent to and parallel with the fault, is analogous to similar structures along Lake fault ends westward in the uppermost Her- lower hinge of a low-amplitude (<1 m), local both the Snowdon and Coal Bank Pass faults, mosa beds near where the Hermosa Group dips . The sandstone dikes indicate dila- suggesting that all of the faults represent a simi- westward beneath the Cutler Group. tion of the limestone bed and injection during lar mechanism, which is interpreted to be domi- Two distinct types of synsedimentary defor- dewatering of the sand. nantly strike slip. mation suggest age constraints for the Little Synsedimentary dip toward the Little Molas Molas Lake fault. Both are expressed in the Lake fault and dilation of brittle limestone near Pattern of Faults upper part of the Hermosa Group, suggesting the fault are consistent with a negative fl ower fault movement during Hermosa deposition. structure along a transtensional segment of a Symmetric anticlines along the Coal Bank Approximately 250 m north of the Little strike-slip fault. The trend of the monocline, Pass, Snowdon, and Molas Creek faults are Molas Lake fault (location 10A, Fig. 2), inter- along which the array of sandstone dikes is consistent with positive fl ower structures along bedded shale and sandstone units in the Hermosa aligned, with respect to the Little Molas Lake strike-slip faults. Fault separation and anticline Group strike northerly, nearly perpendicular to fault is consistent with sinistral strike slip. In amplitude decrease to zero westward along the strike of the fault, and dip westward. One addition to the kinematic implications of the strike. The western ends of the Coal Bank Pass, stratigraphic interval ~1 m thick of thin-bedded synsedimentary structures, two small exten- Snowdon, Andrews Lake, Little Molas Lake, sandstone and shale is deformed by a train of sional faults with no evident synsedimentary and Molas Creek faults defi ne an en echelon asymmetric folds (Fig. 10A) that trend N~45W deformation splay southward from the Little pattern (Fig. 2), which, along with other indica- and verge southwest obliquely toward the Little Molas Lake fault (Fig. 2). tions of strike-slip displacement, is consistent Molas Lake fault. The folds have wavelengths with sinistral slip on the system of faults. of 4–7 m and amplitudes of <2 m. Geometry Molas Creek Fault Zone ranges from open asymmetric to nearly isoclinal REGIONAL IMPLICATIONS recumbent. The overlying beds are planar and The younger (post-Molas), northern branch laterally continuous. Soft-sediment sliding evi- of the Molas Creek fault zone obliquely inter- The time of movement on Ancestral Rocky dently was generated on a southwestward paleo- sects the older (pre-Ignacio), southern branch Mountains faults, primarily the large-scale slope produced either by tectonic movement or that formed the boundary between the Uncom- boundary faults between uplifts and basins, by delta-front deposition; however, the close pahgre Group in the Grenadier fault block and has been constrained generally by the age of geographic association with the Little Molas the Precambrian granite on the north (Fig. 2). prograding, coarse clastic facies of Pennsylva- Lake fault suggests sliding toward the fault, pos- Anastomosing faults bound horses of Pennsyl- nian-Permian age (e.g., Kluth and Coney, 1981; sibly triggered by fault movement. vanian and older stratigraphic units along the Ye et al., 1996; Dickinson and Lawton, 2003). Approximately 75 m south of the Little Molas northern branch of the Molas Creek fault zone. The minimum age of faulting is constrained in Lake fault (location 10B, Fig. 2), one limestone Vertical separation on the post-Molas fault few places, and some or all of the movement

Figure 10. Synsedimentary structures in Hermosa beds near the Little Molas Lake fault. Locations of photographs are shown in Figure 2. (A) Synsedimentary slump folds in sandstone and shale, ~250 m north of the Little Molas Lake fault (view to northwest). Divisions on the rod are 10 cm. (B) Sandstone dikes in limestone bed, ~75 m south of the Little Molas Lake fault (view of limestone bed surface; rod with divisions of 10 cm is oriented N75W).

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on some faults can be inferred to be Laramide East of the kink, the main boundary fault fault within the Grenadier fault block, vertical rather than Ancestral Rockies. Stratigraphic con- (Ridgeway fault) of the Uncompahgre uplift, separation ends along strike in an upright, sym- straints presented here document Pennsylvanian as well as the Sneffels and Grenadier fault metric anticline in a pattern characteristic of a ( Hermosa) movement on some of the faults of the blocks, bends abruptly to the southeast and positive fl ower structure. Folds diverge from the Grenadier fault block, in contrast to a common south, and continues to an intersection with fault in a pattern characteristic of sinistral strike inference that those faults are entirely Laramide the north-striking Pecos-Picuris fault (Fig. 1) slip. Stratigraphic thinning toward the fault and (e.g., Evans, 2002). Because the faults of the (Stevenson and Baars, 1986). In southern intraformational angular unconformities docu- Grenadier fault block end along strike within Colorado, near the intersection with the Pecos- ment growth of the positive fl ower structure dur- the Hermosa outcrops, no post-Hermosa defor- Picuris fault, the northeastern boundary of the ing deposition of the Pennsylvanian Hermosa mation can be documented. The Grenadier fault Uncompahgre uplift is the northeast-directed Group. The timing indicates that the Snowdon block is exposed within the Laramide San Juan Sand Creek–Crestone thrust system (Fig. 1), fault is an Ancestral Rockies structure. Other dome, which has a broadly domal geometry, in which places basement rocks over Pennsylva- Grenadier faults have characteristics consistent further contrast to the steep faults and associated nian-Permian coarse clastic fi ll of the Central with the better-documented history of the Snow- upright folds of the Grenadier fault block. Perm- Colorado trough (Hoy and Ridgway, 2002). don fault. The faults associated with the Grena- ian structure in the Sneffels fault block (north Compression along northwest-striking struc- dier fault block document sinistral strike-slip of the Grenadier fault block) is documented by tures is compatible with sinistral strike slip movement during Ancestral Rockies deforma- an angular unconformity beneath Triassic strata along the east-striking kink. tion, a sense of slip that is consistent with other over a drape fold in the Permian Cutler Group Northwest from the kink, the faults curve to a structures regionally. Similar kinds of documen- (Weimer, 1980). Along the northeastern bound- northwest strike, and most of the displacement tation of time and sense of slip on Ancestral ary of the Uncompahgre uplift, folded angular apparently is transferred to a single boundary Rockies faults are essential to the resolution of unconformities indicate Pennsylvanian-Perm- fault at Gateway, Colorado (Fig. 1) (Steven- regional-scale kinematics and . ian northeast-directed thrusting, and distinct son and Baars, 1986). The boundary fault has Laramide structures overprint the Ancestral been traced farther northwest into eastern Utah ACKNOWLEDGMENTS Rockies structures (Hoy and Ridgway, 2002). (Fig. 1), where seismic refl ection profi les and Along the east side of the Front Range uplift, deep wells document a northeast-dipping thrust Part of this research was supported by a grant from the National Science Foundation, including the fi eld Laramide deformation reactivated some struc- fault at the boundary between the Uncompahgre work of graduate research assistants, Tom Block and tures of Ancestral Rockies vintage (Gerhard, uplift and Paradox basin (Frahme and Vaughn, John Hoover. The ideas expressed here have ben- 1967). Further work is needed to distinguish 1983). A thick succession of Pennsylvanian- efi ted from continuing discussions and fi eld work in Ancestral Rockies faults from Laramide faults, Permian clastic sediment in the footwall beneath the Ancestral Rockies with Don Baars; collabora- which is an essential step to understanding overthrust Precambrian crystalline rocks con- tions in the fi eld with Gene Stevenson, Charles Waag, regional kinematics of the Ancestral Rockies. strains the time of uplift. In a regional context, and Greg Mack; and discussions with Chuck Kluth and Eric Erslev. ArcInfo map compilation and other Sinistral strike-slip movement along part sinistral strike slip on the west-striking system graphics were completed by Mike Solis and Matt Sur- of the southwest-boundary-fault system of the of faults along the Grenadier fault block is con- les. I thank Geosphere reviewers Art Goldstein, Chuck Uncompahgre uplift has important implications sistent with top-to-southwest shortening along Kluth, and an anonymous reviewer. for kinematics of other segments of the same a northwest-striking, northeast-dipping fault to regional structure. The Grenadier and Sneffels the northwest along strike of the regional sys- REFERENCES CITED fault blocks and the faults associated with them tem. The same regional sense of large block dis- Baars, D.L., 1966, Pre-Pennsylvanian paleotectonics—Key indicate the complexity of the boundary-fault placement accounts for both local expressions to basin evolution and petroleum occurrences in Para- system between the Uncompahgre uplift and in the context of along-strike variations in dip dox basin, Utah and Colorado: American Association Paradox basin. The east-west strike of the sys- and strike of the fault surface. of Petroleum Geologists Bulletin, v. 50, p. 2082–2111. 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Mountain Region: Denver, Colorado, Rocky Mountain Hoy, R.G., and Ridgway, K.D., 2002, Syndepositional Association of Geologists, p. 143–165. MANUSCRIPT RECEIVED 7 SEPTEMBER 2006 thrust-related deformation and sedimentation in an Spoelhof, R.W., 1976, Pennsylvanian stratigraphy and paleotec- REVISED MANUSCRIPT RECEIVED 22 DECEMBER 2006 Ancestral Rocky Mountains basin, Central Colorado tonics of the western San Juan Mountains, southwestern MANUSCRIPT ACCEPTED 7 FEBRUARY 2007

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