The Arumbera Sandstone consists of mappable informal units which are repeated in a vertical, cyclic succession. Sandstones of fluvial origin form resistant strike ridges separated by strike valleys, which consist of recessive sandstones and mudrocks of marine origin. Lithofacies 1a, 2b, and 3a are probably of marine origin in intertidal environments. Trace fossil assemblages in lithofacies 3a suggest Skolithos and Cruziana inchnofacies were present. Lithofacies 1e, 2a, 2c, 3b, and 4a are probably of fluvial origin, as the result of coalescing braided stream deposits. The Arumbera Sandstone probably was deposited in a deltaic environment characterized by low wave energy, a micro tidal range, and high input of sand-sized sediment br braided streams. In the western MacDonnell Ranges, the Arumbera overlies the Julie or Pertatataka formations along a sharp but conformable contact. The present upper contact is a low-angle regional unconformity which contains paleotopographic elements that resemble pediments, stripped structural plains, and steep erosional scarps. These paleotopographic surfaces are overlain from east to west by the Chandler, hugh River, and Cleland formations in an onlap relationship. The Arumbera Sandstone is considered part of a molasse sequence associated with the Late Proterozoic and Early Cambrian Petermann Ranges orogeny, which occurred along the present southern and southwestern margin of the Amadeus Basin. The uplifted Petermann Ranges shed detritus from metamorphic, sedimentary, and minor amounts of plutonic rocks. Paleocurrents suggest most terrigenous material was derived from the southwestern margin of the basin. The composition of detrital grains and lack of weathering features in labile detrital grains suggest a hot, semiarid to arid climate in the source area and in the basin of deposition. Sandstone samples examined petrographically primarily are subphyllarenites, subarkoses, arkoses, feldspathic litharenites, and lithic arkoses. The inferred paragenetic sequence is: Eogenetic: (1) mechanical compaction, (2) "dust rims" of hematite, illite, and chlorite, and (3) hematite cement; Mesogenetic: (4) syntaxial feldspar overgrowths, (5) syntaxial quartz overgrowths, (6) hematite cement, (7) carbonate cement, (8) kaolinite replacement, (9) formation of secondary porosity; Telogenetic: (10) chert cement and (11) gibbsite or hematite cement.

Pimple mounds are circular to elliptical domes with basal diameters ranging from 3 to more than 30 m, and heights of 30 cm to more than 2 m above intermound levels. For almost two centuries, the origin of these features has been speculated upon by scientists without general consensus as to one of over 30 different mechanisms suggested for their origin. These soil microfeatures can be observed throughout portions of East Texas as well as Louisiana, Arkansas, Oklahoma, and Missouri. Pimple mounds have been extensively mapped throughout East Texas as complexes covering over 1.0 million ha in 47 soil survey areas. About 600,000 ha are on Pleistocene-age geological formations. This study focused on 5,500 ha in Leon County, Texas, mapped as Rader-Derly complex and Derly-Rader complex. Rader (Aquic Paleustalfs) is on mounds and Derly (Typic Glossaqualfs) in the low intermounds. These soils are mapped primarily on terraces of the Trinity River system within the survey area. Using elevation levels published for the various fluviatile terrace deposits of the Trinity River, six groups (five terrace level groups and an upland group) were identified for analysis of mounds within the study area. Processes and factors of soil formation during the life of these features were considered using two methods ? remotely sensed elevation data and sampling data collected in the field. Size, shape, and relief of mounds were analyzed using airborne-based, remotely sensed LiDAR (Light Detection and Ranging) elevation data. Particle size distributions and pedon descriptions of mounds formed on materials of various ages were compared across the study area with special emphasis given to spatial trends. Analyses indicate a fluvial origin with pimple mound orientation corresponding to surrounding ridge and swale features of the paleoriver. Pimple mounds within the study area formed in the presence of sandy to loamy alluvial sediments and require the presence of accretionary ridge microtopography over point bar deposits. This alluvial parent material and topography were further developed by fluctuations in climate and vegetation over time. The erosional influence of bioturbation by animals and the intense rainfall and flood events which frequent the study area provided an environment in which these soil microfeatures have developed and over time exhibit increased levels of pedogenesis.