Abstract We investigate fault growth and linkage during development of a rafted terrain in the Lower Congo Basin, offshore Angola. Miocene thin-skinned extension has led to the development of isolated raft blocks separated by a graben filled with syn-deformational strata. Angular unconformities together with thinning and onlapping of intra-raft strata onto salt bodies suggest that thick salt was mobile during thin-skinned extension. 3D fault array geometries and displacement patterns record the subsequent deformation history of the graben during further thin-skinned extension. The mode of thin-skinned extension has important consequences for the Neogene turbidite hydrocarbon play associated with the rafted province of the Lower Congo Basin. The presence of thick mobile salt will influence pre-salt source rock maturation and the development of pre-salt/post-salt hydrocarbon migration windows.

Abstract By integrating 3D and 2D seismic interpretation with structural restorations we have reconstructed the evolution of a complex, composite stepped counter-regional salt system in the West Delta/South Pass (WDSP) area of the northern Gulf of Mexico. Biostratigraphically calibrated well data allow the last 10 Ma of the evolution of the salt system to be divided into six stages: (1) sea-floor extrusion of isolated salt tongues fed from the Jurassic Louann salt through northward dipping feeders prior to 7.5 Ma; (2) amalgamation of the salt tongues to form a salt-tongue canopy between 7.5 and 6.4 Ma; (3) counter-regional evacuation of the salt-tongue canopy as a result of enhanced sediment loading due to progradation of the shelf margin between 6.4 and 5.0Ma; (4) evacuation of salt into a series of salt walls linking salt domes between 5.00 and 2.55 Ma; (5) evacuation of the salt walls to form counter-regional fault welds between 1.95 and 0.5 Ma; and (6) final evacuation of most of the salt from deeper levels leaving a series of isolated salt domes connected by counter-regional fault welds. The counter-regional evacuation of the WDSP salt systems illustrates the value and limitations of published 2D models for allochthonous salt, and the reconstructed evolution yields insights into the complex interactions between salt deformation and sedimentation. The results also suggest that the WDSP salt systems significantly affected sediment transport pathways, trap geometries and possibly late stage petroleum migration across evacuating salt welds.

Abstract The purpose of this one-day field trip is to look at the engineering geology and examples of geologic hazards along the western edge of the Denver Metropolitan Area. We will focus on the foothills of the Colorado Piedmont, an area that marks the transition between the Great Plains and the Front Range of the Rocky Mountains. The trip begins and ends in downtown Denver, at the Colorado Convention Center. We will travel northwest to Boulder, then south through Golden and the unincorporated suburbs of Jefferson County, and then return northwest to downtown Denver (Fig. 1). The trip guide contains narrative descriptions for 21 stops, twelve of which we will "roll by" for the GSA Annual Meeting field trip. Because of the dynamic nature of engineering geology, it is useful to look at past events as a means of planning for future development projects. In Colorado, the Colorado Geological Survey (CGS) has a statutory requirement to assist local governments with planning issues involving engineering geology. Since the early 1970s, CGS geologists have reviewed plans for hundreds of subdivisions along the Colorado Piedmont. The authors have provided emergency assistance and advice for incidents involving geologic hazards, as well as research and mapping of general geologic hazards (e.g., Soule, 1978) and specific hazards such as heaving bedrock (e.g., Noe and Dodson, 1997) for the CGS. The goal of this field trip is to provide an account of engineering-geologic factors that have affected the Colorado Piedmont in the past and will continue to do so in the future.

Abstract The field trip will traverse the highly asymmetrical Front Range to examine the west flank's major Laramide thrusts, subsequent volcanic rocks and normal faults, as well as the east flank's higher-angle thrust and reverse faults and their associated fault-propagation folds. Laramide to Holocene tectonics will be debated on the outcrop and during our evening soak at Hot Sulphur Springs.

Abstract The northeastern Front Range provides an excellent test of Laramide tectonic hypotheses. This trip will feature moderate hikes to excellent exposures of basement-involved structures. We will integrate new fault and balancing data into a model for backlimb deformation in basement-cored foreland arches.

Abstract This trip integrates the geomorphic, botanic, and hydrogeologic aspects of diverse ground-water and surface-water systems of the Geneva Creek sub-basin near Guanella Pass, Colorado. Fracture-flow crystalline bedrock systems, and alluvial, colluvial, and glacial systems are integrated for slope and riverine wetland structure and function. Current research regarding the development of the Wetlands Integrated Hydrologic Analysis method for delineating wetlands structure and function in the southern Rocky Mountains is discussed with respect to various sites observed along the field trip route.

Abstract Allogenic and authigenic sediments at Cave of the Winds can be correlated to nearby sediments. The allogenic sediments, which consist of clay, silt and sand, are correlated to the Nussbaum Alluvium. Magnetostratigraphy of the cave sediments combined with the aminostratigraphy of alluvial terraces in the Manitou Springs area indicate that the Nussbaum Alluvium is ~1.9 Ma and that the major episode of sedimentation in Cave of the Winds started ~4.5 Ma and stopped ~1.5 Ma. The authigenic sediments, which consist of iron and manganese oxides, are correlated to similar modern spring sediments. The spring sediments along with the chemistry of the water issuing from the springs of Manitou can be used to show that a mixing zone under Manitou Springs is presently dissolving ~71 tonnes of limestone per year. The cave itself along with the iron- and manganese-rich sediments at the cave indicate that the process of dissolution taking place at the springs is the same process responsible for the dissolution of the cave. Magnetostratigraphic analysis indicates that most of the cave was dissolved from the limestone starting ~4.5 Ma and ceased ~1.5 Ma.

Abstract Loess and eolian sand cover vast areas of the western Great Plains of Nebraska, Kansas and Colorado (Fig. 1). In recent studies of Quaternary climate change, there has been a renewed interest in loess and eolian sand. Much of the attention now given to loess stems from new studies of long loess sequences that contain detailed records of Quaternary glacial-interglacial cycles, thought to be a terrestrial equivalent to the foraminiferal oxygen isotope record in deep-sea sediments (Fig. 2). Loess is also a direct record of atmospheric circulation, and identification of loess paleowinds in the geologic record can test atmospheric general circulation models. Until recently, eolian sand on the Great Plains had received little attention from Quaternary geologists. The past decade has seen a proliferation of studies of Great Plains dune sands, and many studies, summarized below, indicate that landscapes characterized by eolian sand have had dynamic histories.

Abstract George Gaylord Simpson (1902-1984) dominated American paleontology for some five decades spanning the middle of the twentieth century. This dominance was both quantitative and qualitative, for Simpson not only published hundreds of articles, monographs, and books (his bibliography includes more than 750 entries), but his work had major impact on contemporary views of the origin, classification, and evolution of mammals; historical biogeography; principles of taxonomy and systematics; biostatistical methods; and most significantly, the formulation of the modern evolutionary synthesis. The Capitol Heights neighborhood of Denver where Simpson spent his youth includes two of the houses where he lived; the home of his childhood playmate and future wife Anne Roe; the church where he pumped the organ and whose teachings he eventually disavowed; the elementary school where he excelled as a student and suffered for it; the firehouse where the fire horse died; and the corner where he sold lemonade for spending money.

Abstract This field trip presents field evidence for Neogene evaporite tectonism, dissolution of evaporites, and related collapse in Eagle River valley and along the southwestern flank of the White River uplift. In the Eagle collapse center, Pennsylvanian evaporite flowed to form anticlinal diapirs, dissolved, and disrupted a lower Miocene basaltic plateau originally at elevations as high as 3.35 km by tilting, faulting, and sagging to elevations as low as about 2.1 km. Also in the Eagle collapse center, the 30 x 10-km, homoclinal Hardscrabble Mountain sank into evaporite during Triassic and Permian collapse followed by Neogene(?) tilting and collapse, based on seismic reflection data. Along the southwestern flank of the White River uplift in the northwestern part of the Carbondale collapse center, parts of the Grand Hogback monocline have collapsed northeastward toward a series of strike-elongate extrusive diapirs. The volume of evaporite removed from the Eagle and Carbondale collapse centers during the Neogene (about 2,250 km 3 from an area of roughly 4,500 km 2 ) was calculated by measuring the departure of collapsed basalts from an assumed original basalt plateau. Regional Neogene uplift and incision of the Rocky Mountains, which locally began about 8-10 Ma, probably triggered dissolution and collapse. Presently the Colorado River removes a dissolved-solids load of about 1.4 x 10 9 kg per year from the two collapse centers.

Abstract This two-day excursion will travel to the Yampa coal field located in parts of Moffat, Rio Blanco, and Routt Counties, northwestern Colorado. The excursion will visit classic regression/transgression successions in the Upper Cretaceous coal-bearing Mesaverde Group, which was deposited along the western edge of the Cretaceous seaway. It includes visits to inactive and active mine sites where past and current mining practices will be discussed. This guide summarizes the stratigraphy, sedimentology, and coal geology of the Yampa coal field. The trip will emphasize the depositional setting, sedimentology, and quality of the Upper Cretaceous coals and coal-bearing strata of the Mesaverde Group.

Abstract This guide consists of three general sections: an introduction that includes discussions of Raton basin stratigraphy and the Cretaceous Tertiary (K-T) boundary; descriptions of the geology along the route from Denver, Colorado, to Raton, New Mexico; and descriptions of several K-T sites in the Raton basin. Much of the information is from previous articles and field guides by the authors together with R. M. Flores and from road logs co-authored with Glenn R. Scott, both of the U.S. Geological Survey.