The Woods Hollow Shale (Middle Ordovician), Maravillas Chert (Late Ordovician), Caballos Novaculite (Devonian?), Santiago Chert (Devonian?) and Mississippian shales of the Tesnus Formation are exposed along the Horse Mountain anticline 19 1/2 miles south of Marathon on U. S. Highway 385. Horse Mountain anticline is a narrow, northeast trending fold on the southeast flank of the complex anticlinorium which was formed by pulsatory thrusting and folding during the Mississippian and Pennsylvanian Periods. It is within the Marathon salient of the Ouachita system. All structures are related to a northwest-southeast maximum principal stress. The chert and novaculite folded concentrically above detachment planes in the Woods Hollow Shale. Subsequent failure of the shale produced a syncline along the crest of the anticline. This was followed by major thrusting over the northwest limb. The deformation culminated with underthrusting along the axis and southeast limb, right-handed and left-handed strike-slip oblique faulting and transverse faulting. Erosion has left only the nearly vertical, resistant beds on the limbs and a vestige of the former crestal syncline. Two sets of shear fractures (N 47° W, 23° SW; N 81° W, 23° NE) are perpendicular to the beds and make acute angles with the local maximum principal stress along the axis of the fold. A third set of extension fractures (N 52° W, 79° SW) is nearly perpendicular to the axis of the fold and formed when the flanks of the fold dipped 45 degrees. / text

Ordovician through Pennsylvanian strata of the Ouachita and Marathon sedimentary sequences show correlated Nd-Sr isotopic relations, Th/Sc ratios and REE patterns typical of evolved upper crustal sources, indicating that the 2000 km long Ouachita-Marathon fold belt consists primarily of recycled crustal materials. wi thin this sequence, Nd isotopes distinguish three distinct provenance groups: 1) Lower to Middle Ordovician hemipelagites and quartzose turbidites with ε(Nd)(t) = -13 to -16 (T(DM) = 1.8 to 2.1 Ga); 2) Upper Ordovician through Pennsylvanian hemipelagites and quartzolithic turbidites with ε(Nd)(t) = -6 to -10 (TOM = 1.4 to 1.7 Ga); 3) Mississippian tuffs with ε(Nd)(t) = -2 (TOM = 1.1 Ga). These data record a rapid Ordovician (ca. 450 Ma) shift in sedimentary sources within the off-shelf passive-margin sequence of deep-marine cherts and shales. Ouachita Silurian turbidites (ε(Nd) = -7 to -8) are isotopically identical to Middle Ordovician Taconic turbidites of the Sevier basin (Tellico Formation) in eastern Tennessee (ε(Nd) = -7 to -8), suggesting that Appalachian clastic wedges supplied Ouachita deep-sea turbidites beginning in the Late Ordovician. Pennsylvanian non-marine sandstones and shales from the Arkoma, Illinois, and Black Warrior basins have ε(Nd) = -7.5 to -10.0, similar to the thick (>10-12 km) Ouachita Carboniferous turbidite flysch sequence (ε(Nd) -7.5 to -9.6). The remarkable isotopic homogeneity of sediments delivered to the Ouachita-Appalachian region over this period implies extremely effective mixing and dispersal processes on a large (continent-wide) scale, consistent with a collisional belt provenance. A long-lived (ca. 150 Ma) tectonic link between the Appalachians and Ouachitas is thus implied by these data. Mississippian silicic ash-flow tuffs have trace-element and Nd isotopic compositions consistent with a continental-margin arc source. The active volcanic arc which erupted these tuffs apparently extended at least 1000 km from the Ouachita region to south of the Marathon region, but did not supply a significant component of the flysch. The data are consistent with submarine fan models of Ouachita flysch sedimentation demonstrating dominantly longitudinal transport down the axis of a Carboniferous remnant ocean basin from sources to the east. A model is proposed for the evolving Ouachita-Marathon suture between Laurentia and Gondwana, expanding upon Graham et a1. (1975), whereby dominantly Appalachian-derived seafloor detritus was swept up along the flanks of an approaching arc-trench system into sUbduction complexes and recycled incrementally along the length of the collision zone into the Marathon region.

Lower Cretaceous strata in central Texas are divisible into genetically related rock bodies. The Lower Trinity Group contains a lower terrigenous near shore deposit (Hosston Formation) which interfingers with an overlying shallow water, offshore limestone-dolomite sequence (Sligo limestone); this Group may contain a barrier reef section downdip. The Middle Trinity Group, a sequence of grey to black shale and limestone, does not intercalate with either the underlying or overlying rock bodies. Updip, near the Texas craton hingeline, it is separated from the overlying rocks by a disconformity. Downdip the Stuart City reef, a rudistid limestone, exists continuously between the top of the Middle Trinity Group and the upper part of the Georgetown formation of the Washita Division. Back reef deposits are separable into Upper Trinity Group, Fredericksburg Group and part of the Washita Division. The Upper Trinity Group consists of the basal Hensel sand (mostly updip outside the area of study) which intercalates going downdip with the usually dolomitic Glen Rose limestone. The middle part of the lower Glen Rose contains a reefal limestone section which is restricted to the area near and over the San Marcos arch and near the Stuart City reef. The upper Glen Rose is a sequence of dominantly evenly-bedded, dolomitic, foraminiferal limestone. The Fredericksburg Group includes the East Texas basin lagoon deposits of the Walnut and Comanche Peak formations and the rudistid limestone deposits of the lower Edwards. The lower Edwards intercalates with the lower units of the Walnut over the San Marcos arch and with successively higher units toward the East Texas basin; lower Edwards and Comanche Peak produce a similar pattern. The basal unit of the Washita Division is a widespread black shale (Kiamichi) and shaley limestone (middle (Kiamichi) Edwards) which is absent by onlap around the Belton high (proposed new term). The dolomitic, rudist limestone of the upper Edwards interfingers with the lower beds in the Georgetown limestone north of the San Marcos arch. The remainder of the Georgetown and the overlying Del Rio clay are laterally continuous throughout the area of study. The Buda limestone pinches out near the edge of the Texas craton and thickens going southeast and downdip.

Detailed mapping of Carboniferous rocks westward from areas mapped by Bogardus and Oden shows that rock units within the Marble Falls formation are traceable across "Cavern Ridge" a "barrier" invented by Plummer and referred to by others. The Ives breccia, Chappel limestone, Barnett formation, and Lower member of the Marble Falls formation formed as a transgressive depositional sequence. A thin zone of bypassing separates the Barnett and Marble Falls, but a hiatus between them in the Hall area cannot be demonstrated. A small-scale disconformity exists between the Lower and Middle members of the Marble Falls. Local faulting during deposition of the Marble Falls produced the Gibbons conglomerate, influenced the accumulation of Middle Marble Falls shale, and elevated the "Hall Uplift." Some lithosomes in the Upper member of the Marble Falls pinch out near the "Hall Uplift." Sandstone and mudstone of the Strawn formation abut against the carbonate mass of the Marble Falls to the north, but whether a period of erosion intervened between the deposition of the two is not known. Analysis of seven species of corals, twenty-three of brachiopods, two of pelecypods, five of gastropods, two of cephalopods, two of trilobites, and fourteen of conodonts suggests that the Chappel, Barnett, and Marble Falls faunal assemblages most closely resemble those of the Chouteau formation, Caney formation, and Morrow group respectively. In the Hall area no part of the Marble Falls should be correlated with the Atoka series. / text

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The Llano Uplift of central Texas contains the largest exposure of Mesoproterozoic rocks along southern Laurentia and is thus crucial to the understanding of orogenesis and plate reconstructions along a portion of one of the largest orogens in the world. Most of the current understanding of the Mesoproterozoic tectonic evolution of southern Laurentia comes from the southeastern portion of the Llano Uplift. To fully characterize the tectonic evolution Llano Uplift, detailed mapping is necessary in the less-studied western Llano Uplift. The Mesoproterozoic Llano Uplift exposes mid-crustal, poly-deformed and metamorphosed schists and gneisses and abundant pre- to post-tectonic granites through an erosional window of Phanerozoic sedimentary rocks. Three lithologic groups were mapped in the western Llano Uplift, from structural highest to lowest these are the Valley Spring Gneiss (VSG), Lost Creek Gneiss (LCG) and Packsaddle Schist (PS). The VSG consists of pelitic schists and pink quartzofeldspathic schists and gneisses. The LCG is a thick, homogeneous package of medium- to coarse-grained augen granite gneiss, interpreted to be a deformed, coarse-grained, porphyritic pluton. The PS consists of a heterogeneous package of interlayered quartzofeldspathic gneisses, amphibolites and minor marbles. These lithologies are consistent with the PS and VSG domains described in the southeastern Llano Uplift (Mosher, 1998; Reese et al., 2000). The exotic Coal Creek Domain (CCD) of the southeastern Llano Uplift is not observed in the western Llano Uplift. The western Llano Uplift, including the VSG, LCG and PS, records a deformational history that resulted in multiple fold generations (F1-F5) and is characterized by a penetrative axial planar foliation (S1-S5). F2s are isoclinal folds of S0 (primary layering) and S1 that locally fold F1 axial planes and have steeply plunging and generally easterly trending hinge lines. F3 folds are locally developed, nearly colinear and coplanar with F2s, tight to open, and fold all previous structures (F1/F2) and fabrics (S1/S2). F4s are open folds with northeast-trending axial traces that occur on a regional-scale. F5s are open to tight folds of all previous structures, with hinge lines that are primarily southeast trending and steeply plunging. S0 to S3 orientations vary from north to east dipping because of reorientation by younger folds. S4 foliations strike to the northeast and S5 foliations are northwest striking and nearly vertically dipping. Late left-lateral shear zones (D6) with generally an easterly trend and boudinage affects the VSG, LCG and VSG in this study area and is commonly associated with unfoliated granite material. Four generations of intrusive granitic sills and dikes are documented and provide relative and absolute age constraints on deformation. The oldest recognized deformation (D1-D3) is constrained between 1253 +5/-3 Ma and 1126 +5/-4 Ma (Roback, et al., 1999). D4 and D5 deformation are constrained between 1126 +5/-4 Ma and 1076 ± 5 Ma (Roback, et al., 1999). Although a change in metamorphic conditions is documented to have occurred between D2 and D3, metamorphic fabrics and assemblages indicate granulite facies conditions during D1, D2 and D3. Amphibolite facies metamorphism occurred during D4 and presumably D5. Deformation in the eastern Llano Uplift has a similar polyphase deformational history to that recorded here for the western Llano Uplift. Deformation in the eastern Llano Uplift is similarly constrained between ca. 1238 to 1091 Ma. In addition, the youngest fold generation (F5) can be directly correlated in orientation and timing from the western to the eastern Llano Uplift, and is constrained between ca. 1119 and 1091 Ma in the eastern uplift. Both the western and eastern Llano Uplift contain late shear zones and extensional structures. Structural differences between the western and eastern Llano Uplift include differences in style and orientation of all but the latest (D5 and D6) structures. In addition, dip of fabrics and, therefore, structural stacking of lithologic domains is opposite, and no mylonite zones were identified in the west. In conclusion, the lithologic domains appear to correlated across the Llano Uplift based upon gross lithologic similarities and the tectonic evolution is similar to the well-studied eastern Llano Uplift, though the kinematics and orientations differ. These conclusions may require that the kinematics of deformation in the southeastern uplift were controlled by the presence of the exotic island arc terrane (CCD) whereas the kinematics of deformation in the western uplift were controlled by continent-continent collision. / text