Journal Of Structural Geology - Programs In Geology .

-8000'contacts; dotted whereconcealed or inferredCambrian Dagger Flat Ss.Ordovician Marathon Ls.OrdovicianFt. Pena Fm.& Alsate Sh.lower Paleozoic Slope/Rise SedimentsMississippian Tesnus Fm.Permian Cathedral Mtn.Fm. & Hess Ls. of 0'-4000'Pennsylvanian GapTank Fm. & Dimple Ls.Ellenberger Simpson Mobil OilAdams No. 1(projected)Dev. & Sil.Montoyabend insectionWoodford Sh.StrawnS.L.4000'undifferentiatedCretaceous marinesedimentslower Paleozoic marineshelf sequenceDevonianCaballosNovaculiteLightning Flat Horse Mtn.GrangerAnticlinebend insectionkDugout Cree ThrustGlass Mtns.Ordovician WoodsHollow Shale& Maravillas Chertfault; dashed whereconcealed or inferredDevil’sBackbone Th.Hell’s HalfAcre Th.Arden DrawThrustHaymondSynclinePena BlancaThrust (proj.)Warwick Th.(proj.)Marathon RoofTh. (postulated)Marathon Anticlinoriumoffset in sectionColorada Synclinoriumbend in sectionThis study is based on a 49 km long by 7 km wide transectextending in a generally northwest–southeast direction across theMarathon inlier (Fig. 3). Within this corridor, the surface geologicmapping of King (1937) and limited existing well data were augmented by detailed structural measurements, and additionalmapping. With these data we constructed a regional cross sectionacross the orogen (Fig. 4). The earlier studies demonstrated that thestyle of contraction within the Marathon region is markedly different that that observed in many other more ‘‘typical’’ thrust belts.Apparent extreme internal shortening within mechanically incompetent units precluded the use of typical cross sectional balancing techniques that employ fault-bend fold or fault-propagationfold geometries. Thus, we attempted to balance our section usingcompetent units, such as the Caballos Novaculite, as the control onoriginal line length. Incompetent units were then approximatelyarea balanced (Mitra and Namson, 1989). As with all sections, oursis most certainly not correct in detail, but represents one viablesolution of many.Several aspects of the cross section are striking and some of thefeatures set the Marathon thrust-fold belt apart from better knownthrust belts such as the Appalachians or Canadian Rockies. Thecross section clearly indicates that tectonic transport of the Marathon facies rocks has been tens of kilometers and possibly muchlarger. Good evidence in the form of map patterns and constraintsof geologic balancing indicate that the basal Dugout Creek thrust istightly folded. This implies substantial progressive internal deformation of the allochthonous rocks and the underlying latePaleozoic foredeep deposits.The southeast part of the transect crosses imbricate thrustscutting Tesnus Formation at the surface. Along strike these thrustscut strata ranging from Woods Hollow Shale through HaymondFormation. The central part of the transect crosses the Dagger Flatand Marathon anticlinoria which consist of imbricates andduplexes involving early and middle Paleozoic strata. Finally, thenorthern part of the transect crosses folded and thrusted strata ofthe late Paleozoic Gaptank Formation that were deposited in theforeland of the thrust belt. At the level of surface exposure, much ofthe internal shortening of the allochthonous rocks is by tightfolding rather than major imbrication. Internal shortening of theserocks by both folding and thrusting is at least 80% and could bemuch greater. Extreme shortening across major anticlinoria resulted in the development of upper level detachments and duplexzones. Toward the interior of the belt (southeast), where latePaleozoic strata are widely exposed, imbricate thrusting appears tobe more important. However, this may be a perception based on thestructural depth exposure.Dagger Flat Anticlinoriumbend in section3. Structure-4000'Cretaceous4000'The Caballos Novaculite strongly influences the structural styleof the belt because it forms a thick, competent member within theotherwise thin-bedded, mechanically weak pre-tectonic part of theMarathon sequence. It is involved in the formation of both detachedbuckle folds and fault-propagation folds.In the northeastern part of the Marathon inlier, outside thearea of this study, thrust sheets involving only the late Paleozoicsyn-tectonic formations are exposed at the surface (Fig. 3). Welland seismic data (Reed and Strickler, 1990) suggest that this relationship is due to a north-northwest-trending lateral rampnortheast of the Dagger Flat and Marathon anticlinoria alongwhich the basal thrust ramps upward to the Woods Hollow Shaleto the northeast. Lacking multiple decollement horizons, thestructure of this northeastern area is dominated by a series oflarge fault-propagation folds and adjacent synclines more reminiscence of ‘‘typical’’ thrust belts (Rodgers, 1990; Mitra, 1992).903S.L.R.G. Hickman et al. / Journal of Structural Geology 31 (2009) 900–909Fig. 4. Cross section across the Marathon region.3.1. Structure of the interior part of the beltThe most interior part of the thrust belt is exposed southeast ofthe Peña Blanca Mountains (Fig. 3). Surface exposures are largelyof folded Tesnus Formation. Exposed structures are chiefly opensynclines that are cut respectively on their southeast flanks bythree southeast-dipping thrusts, the Devil’s Backbone, Hell’s HalfAcre, and Arden Draw thrusts (Fig. 4). In the area of the crosssection the Devil’s Backbone thrust consists of two imbricateswith a series of folds in the sliver between the thrusts. The HorseMountain anticline is interpreted to be a transported faultpropagation fold developed above the Arden Draw thrust (Fig.1).The Hell’s Half Acre thrust is developed on the backlimb of theHorse Mountain anticline. In the eastern part of the Marathon

904R.G. Hickman et al. / Journal of Structural Geology 31 (2009) 900–909inlier, Muehlberger et al. (1984) recognized that the Hell’s Halfacre thrust is an out-of-sequence feature since blocks of Pennsylvanian Dimple and Haymond derived from underlying thrustsheets or folds occur in the fault zone.Locally, a cleavage parallel to the axial surfaces of folds is developed within shaly layers of the Tesnus Formation (Fig. 5). On theflanks of Horse Mountain and other folds a pencil cleavage (Ferrill,1989) formed by the intersection of this cleavage with bedding isdeveloped in the Tesnus. This lineation parallels the plunge of thefolds. These cleavages indicate that these strata have undergonesignificant strain (Hobbs et al., 1976).The deep structure of this part of the thrust belt is poorly known.Based on the involvement of Caballos Novaculite and Woods Hollow Shale in the Horse Mountain anticline, it is inferred that allthree of the thrusts join a decollement near the base of the AlsateShale. In the cross section (Fig. 4) the Caballos and older formationsFig. 5. (A) Axial plane cleavage developed in pelitic layer of the Tesnus Formation onthe flank of Granger anticline (see Fig. 4 for location); (B) Pencil cleavage lineationdeveloped in shales of the Tesnus Formation of the Peña Blanca Mountains. Pencils areparallel to the plunge of regional folds.above the thrusts are portrayed as being largely unfolded. However,because these units are folded everywhere that they are exposed, itis likely that they are folded into relatively small-scale folds atdepth.3.2. Structure of the central part of the beltAt the surface, the central part of the thrust belt consists of twoanticlinoria of complexly deformed pre-tectonic Marathon faciesstrata, separated by a synclinorium of Tesnus Formation (Fig. 4).3.2.1. Dagger Flat anticlinoriumThe Dagger Flat anticlinorium is most southerly of the complexly deformed zones. The flanks of the anticlinorium are markedby resistant ridges of steeply dipping Caballos Novaculite. The coreof the structure consists of folded and imbricated Ordovician andCambrian strata. The folds are tight, high-amplitude chevron foldswith wavelengths of 150–300 m. All dips in the core of the fold aresteep (Fig. 4).Exposures to the northwest, and wells indicate that the DugoutCreek thrust, which is the basal thrust involving Marathon strata inthis region, dips southward beneath the Dagger Flat anticlinorium.This makes it likely that the exposed imbricate thrusts ramp upward from the Dugout Creek thrust. Down-structure viewing ofmapping in the Warwick Hills area (King, 1937) at the northeasterlyplunge of the anticlinorium reveals that the imbricates are structurally overlain by tightly folded thrust sheets composed of theentire Marathon sequence that are detached in the Lower Ordovician shales (Fig. 6). An integration of the subsurface, surface, andprojected down-plunge geology shows the overall structure of the‘‘anticlinorium’’ to be a complex series of folded duplexes (Fig. 4B).The folded thrusts of the Warwick Hills area are the roof thrusts ofthe highest duplexes. High-relief detached folds above the roofthrusts accommodate shortening at a shallower level. Local shortening of the strata of the roof thrusts by thrusting and folding wasextreme and may exceed 80%. Deformation of these roof thrustswas progressive because the Warwick Hills thrust is less folded thatthe underlying unnamed thrust (Fig. 4B).The southeastern flank of the Dagger Flat anticlinorium isbounded by left-stepping, high-relief folds involving the WoodsHollow Shale through lower Tesnus Formation. The transectcrosses this fold train in the area of the Peña Blanca Mountains(Figs. 3 and 7). At the surface these folds are detached in theWoods Hollow Shale and are cut by thrusts (Figs. 4B and 7). Thefolds have wavelengths of about 450 m and have amplitudes of150–700 m. The shape of the folds is controlled by the CaballosNovaculite which acts as a thick competent layer within a sequence of thin-bedded shales, cherts and limestones. The CaballosNovaculilte is typically folded into a concentric shape, while in thecores of anticlines the Maravillas Chert and Woods Hollow Shaleare deformed into smaller wavelength box- and chevron-shapedfolds (Fig. 8).The thrusts cut across the competent Caballos Novaculite at highangles (60–90 ) suggesting that the folds initially developed asdetached buckle folds and, after significant shortening, were cut bythrusts. Because strata older than the Woods Hollow Shale are notexposed, these thrusts are interpreted to sole above the MarathonLimestone (Fig. 4B). Map patterns show that the associated thrusts

#147; 2, Mobil Oil Adams #1; 3, Gulf #1 Combs; 4, Turner #1 Combs. 902 R.G. Hickman et al. / Journal of Structural Geology 31 (2009) 900–909 The Ca