Tectonics of Central Texas

Abstract

A clearer concept of the growth of the Llano uplift and related principal structural features of central Texas is sought through use of the present increased store of surface and subsurface geological data. Generalized thickness maps have been prepared as an aid to analysis of time, place, and amount of major uplifts and subsidences, some of which were intense, localized tectonic movements, others widespread epeirogenic adjustments. The most obvious major tectonic feature in central Texas is the Llano uplift where the pre-Cambrian basement complex crops out in an area 40 miles wide and 70 miles long. During much of early Paleozoic time this area was a seaway and for long intervals thereafter it was alternately above and below sea-level. Major diastrophic changes occurred after Early Ordovician and before Late Pennsylvanian, by which latter time the pre-Cambrian surface had become at least 10,000 feet higher in the western part of the Llano uplift than beneath the flanking Fort Worth and Kerr basins. Erosional losses indicate that uplift accounted for about one-third and subsidence two-thirds of the vertical movement in these structural adjustments. Many pronounced structural features observed in the Llano uplift show a surprising lack of parallelism with the main northwest axial trend. Instead, a north to north-northeast trend is common, thereby dividing this uplift into several major segments which are bounded by relatively steep dips and extensive normal faults with vertical displacement as great as 3,000 feet. These major intersecting tectonic features appreciably affected only those beds that are older than the Lazy Bend group (restricted) of the Strawn series. Thereafter structural trends developed mainly along northwest trends. Mississippian outcrops in the Llano region transgress the truncated Ordovician Ellenburger group. Drilling has shown an increasing loss of section west of the Llano uplift so that, as a result of both erosion and non-deposition, Upper Pennsylvanian (Canyon) marine sediments locally overlap Cambrian rocks in and near northeast Menard County. Farther west and northwest, Middle Pennsylvanian beds rest on truncated Mississippian and Ordovician or older rocks in a large region, heretofore called the Concho arch, where local as well as regional tectonic features had developed mainly along trends varying from north-northeast to northwest. Thin Middle Pennsylvanian marine sediments of the Lampasas and Strawn series deposited across this base-levelled region are chiefly limestones and shales o the platform type in contrast to thick basinal type deposits on the east and south. The northwest part of the Llano area evidently remained somewhat above sea-level during most or all of the Strawn time. However, at least 2,000 feet of Upper Pennsylvanian and Lower Permian sediments are assumed to have been deposited across the Llano region, judged by their thickness and marine character, also by projection of regional dip, in near-by outcrops. Regional upward tilting toward the east, followed by extensive erosion, has brought prominence to the Llano uplift. As disclosed by drilling, a regional downward tilting toward the west has given the eastern part of the Concho platform the appearance of a major arch, commonly referred to as the Bend arch or flexure. The much larger Concho arch and broad Concho platform between the Bend axis and the Midland basin lost much of their original dominant position as a result of this regional tilting which began in Late Pennsylvanian, but took place mainly during Permian time. Subsequently, epeirogenic movements, including continental emergence after the Lower Cretaceous epoch, have permitted erosion of all sediments and re-exposure of pre-Cambrian basement in the Llano area, now known geogr phically as the Central Mineral region.