Historical research reveals a repeating pattern of uplift, erosion, and deposition in the region of the Llano Uplift, central Texas. This report examines the topography of the pre-Cretaceous landscape. The data consist of points, in three dimensions, that are located on the erosional surface, as determined by three methods. Category I data lie upon the contact between Cretaceous strata and underlying Paleozoic sediments or Precambrian basement; Category II data are defined in the subsurface from well logs; and Category III data are topographic high points where the Cretaceous has eroded away, but the underlying unit has not eroded (an exhumed surface). Digital mapping procedures were used to create triangulated irregular networks, three dimensional scenery, and topographic profiles. The digitally reconstructed surface is compound, consisting of higher, older erosional surfaces, incised into by rejuvenated stream activity to create lower, younger surfaces. This valley/divide topography, which is regional in extent, could not have been visualized without modern GIS technology. / text

A new barometric technique measuring stored stress in quartz inclusions via laser Raman microspectrometry was employed in an attempt to elucidate the extent of highpressure (HP) metamorphism in the Llano Uplift of central Texas. Rare lithologies within the Llano Uplift contain mineralogical evidence of HP metamorphism (pressures from 1.4 to 2.4 GPa at temperatures from 650 to 775°C), but much of the uplift is composed of felsic gneisses lacking any HP signature; these felsic gneisses may never have transformed to HP assemblages, or they may have been thoroughly overprinted by later low-pressure events. Barometry via laser Raman microspectrometry computes entrapment pressure for a quartz inclusion in garnet from measurement of the displacements of its Raman peak positions from those of a quartz standard at atmospheric pressure. Quartz inclusions in garnets that grew in felsic gneisses under HP conditions should retain HP signatures, despite later overprinting. Application of the Raman microspectrometry technique should therefore allow barometry of previously uncharacterizable rocks. For two localities in the Llano Uplift, entrapment pressures from Raman barometry (0.6-0.7 GPa and 0.2-0.3 GPa) were substantially lower than pressures expected based on conventional barometers (1.4 GPa and 1.6-2.4 GPa). This absence of any HP signatures in the Llano rocks contrasts with more successful applications of the Raman technique by previous workers in high P/T blueschist-facies rocks. A key difference in the Llano rocks is that they reached peak temperatures at which intracrystalline diffusion in garnet, driven by compositional gradients produced during growth, had noticeable effects: complete homogenization of growth zoning had occurred in the locality that produced the greatest discrepancies between Raman and conventional pressures, and modest relaxation of zoning occurred in the locality with the smaller discrepancies. The failure of the Raman technique to recover pressures consistent with conventional barometry in the Llano Uplift is therefore attributed to relaxation of stress on the quartz inclusions as the result of intracrystalline diffusion within the garnet. This conclusion suggests that use of the Raman barometric technique must be restricted to rocks whose time-temperature histories produce only very limited intracrystalline diffusion in garnet, typically those rocks whose peak metamorphic temperatures fall at or below upper amphibolite-facies conditions. / text

The Pennsylvanian Marble Falls Formation in the Llano Uplift region of the southern Fort Worth Basin (Central Texas) is a Morrowan-Atokan mixed carbonate-siliciclastic unit whose deposition was influenced by icehouse glacioeustatic sea-level fluctuations and foreland basin tectonics. Previous interpretations of the Marble Falls Formation focused on outcrop data at the fringes of the Llano Uplift. This study uses a series of 21 cores to create a facies architectural model, depositional environmental interpretation, and regional sequence stratigraphic framework. On the basis of core data, the study area is interpreted to have been deposited in a ramp setting with a shallower water upper ramp area to the south and a deeper water basin setting to the north. Analysis of cores and thin sections identified 14 inner ramp to basin facies. Dominant facies are: (1) burrowed sponge spicule packstone, (2) algal grain-dominated packstone to grainstone, (3) skeletal foraminiferal wackestone, and (4) argillaceous mudstone to clay shale. Facies stacking patterns were correlated and combined with chemostratigraphic data to improve interpretations of the unit’s depositional history and form an integrated regional model. The Marble Falls section was deposited during Pennsylvanian icehouse times in a part of the Fort Worth Basin with active horst and graben structures developing in response to the Ouachita Orogeny. The resulting depositional cycles reflect high-frequency sea-level fluctuations and are divided into 3 sequences. Sequence 1 represents aggradational ramp deposition truncated by a major glacioeustatic sea-level fall near the Morrowan-Atokan boundary (SB1). This fall shifted accommodation basinward and previously distal areas were sites of carbonate HST in Sequence 2 deposition following a short TST phase. Sequence 3 represents the final phase of carbonate accumulation that was diachronously drowned by Smithwick siliciclastics enhanced by horst and graben faulting. These findings contribute to our understanding of the depositional response to glacioeustatic sea-level changes during the Pennsylvanian and can also form the basis for constructing a sedimentological and facies analog for Morrowan to Atokan shallow- to deepwater carbonates in the Permian Basin and the northern Fort Worth Basin. / text

Deformation and partial melting have long been recognized to occur together, but differentiating which actually occurred first has remained enigmatic. Prevailing theories suggest that partial melting typically occurs first, and deformation is localized into melt-rich areas because they are rheologically weak. However, evidence from three different areas, suggests the role of strain has been underestimated in localizing partial melting. The Wet Mountains of central Colorado provide evidence for synchronous partial melting and deformation, with each process enhancing the other. Throughout the Wet Mountains, deformation is concentrated in areas where melt producing reactions occurred, and melt appears to be localized along deformation-related features. Melt microstructures present within the Wet Mountains correlate well with crustal-scale plutons and magmatic bodies and provide a proxy for crustal-scale melt flow. Granitic gneisses from the Llano Uplift, central Texas, provide evidence for partial melting occurring within small-scale shear zones and surrounding country rocks, synchronously. In the field, shear zones appear to contain former melt, whereas the country rock does not provide macroscopic evidence for partial melting. However, detailed microstructural investigation of shear zones and country rocks indicates the same density of melt microstructures, in both rock types. Melt microstructures are important for understanding the full melting history of a rock and without detailed structural and petrographic analysis, erroneous conclusions may be reached. Granulite-facies migmatites of the Albany-Fraser Orogen, southwestern Australia, have undergone partial melting, synchronous with three phases of bidirectional extension. Four major groups of leucosomes, including: foliation-parallel, cross-cutting, boudin neck and jumbled channelway leucosomes and late pegmatites were analyzed via whole-rock geochemistry, and there is evidence for fluid-saturated and -undersaturated biotite- and amphibole-dehydration melting. Migmatites from these three locations contain pseudomorphs of melt along subgrain and grain boundaries, areas of high dislocation density, in quartz and plagioclase. For these rocks that involve multicomponent systems, the primary cause for preferential melting in high strain locations is enhanced diffusion rates along the subgrain boundary because of pipe diffusion or water associated with dislocations. / text