Abstract The Witwatersrand Basin is an Archaean basin situated on the Kaapvaal Craton of Southern Africa. The results presented here focus on the structural geometry and development of the basin. Detailed structural sections across the western and northwestern parts of the basin are presented, using seismic data integrated with borehole, mine and outcrop information. The structural development of the basin can be expressed in simple terms, and in a manner that is standard practice within the oil industry. There are several clearly identified stages in basin evolution. The basin was initiated as a rift during Dominion Group times ( c. 3074 Ma), with post-rift thermal subsidence during the early part of West Rand Group times. Thermal subsidence would have been completed by late West Rand Group times ( c. 2900 Ma). Minor volcanic interludes within the West Rand Group sequence may testify to the existence of phases of extension during West Rand Group times. Onset of compression and thrusting outside the thermal basin generated a flexural load and clastic input into the basin as it evolved from thermal sag to foreland basin during late West Rand Group times ( c. 2950 Ma). Foreland basin development culminated in Central Rand Group times, with an increasingly coarse-grained clastic input, and thrust systems that progressively encroached on the basin margins, profoundly influencing structural styles. Thrusting was interrupted during Klipriviersberg Group times (2714 Ma) by the accumulation of basic volcanic rocks. Further thrusting occurred at the end of Klipriviersberg Group times. The basin then returned to an extensional tectonic setting during the Platberg Group rifting (2709 Ma), a major rift event across much of the area. Thermal subsidence related to this rift phase occurred during late Platberg Group times. The overprint of Platberg Group extensional faults breaks up the structural continuity of the Central and West Rand Group sediments within, and adjacent to, the basin, and it is difficult to elucidate the earlier compressional thrust-fold structures. Careful integration of seismic, borehole, mine and outcrop data allows structures from the thrusting event be recognized and more complete sections drawn. These present a larger-scale view of the structure of the basin than has previously been obtained.

ABSTRACT This contribution is concerned with the debated origin of the impact melt rock in the central uplift of the world’s largest confirmed impact structure—Vredefort (South Africa). New major- and trace-element abundances, including those of selected highly siderophile elements (HSEs), Re-Os isotope data, as well as the first Se isotope and Se-Te elemental systematics are presented for the felsic and mafic varieties of Vredefort impact melt rock known as “Vredefort Granophyre.” In addition to the long-recognized “normal” (i.e., felsic, >66 wt% SiO 2 ) granophyre variety, a more mafic (<66 wt% SiO 2 ) impact melt variety from Vredefort has been discussed for several years. The hypothesis that the mafic granophyre was formed from felsic granophyre through admixture (assimilation) of a mafic country rock component that then was melted and assimilated into the superheated impact melt has been pursued here by analysis of the two granophyre varieties, of the Dominion Group lava (actually meta-lava), and of epidiorite mafic country rock types. Chemical compositions, including high-precision isotope dilution–derived concentrations of selected highly siderophile elements (Re, Os, Ir, Pt, Se, Te), and Re-Os and Se isotope data support this hypothesis. A first-order estimate, based on these data, suggests that some mafic granophyre may have resulted from a significant admixture (assimilation) of epidiorite to felsic granophyre. This is in accordance with the findings of an earlier investigation using conventional isotope (Sr-Nd-Pb) data. Moreover, these outcomes are in contrast to a two-stage emplacement model for Vredefort Granophyre, whereby a mafic phase of impact melt, derived by differentiation of a crater-filling impact melt sheet, would have been emplaced into earlier-deposited felsic granophyre. Instead, all chemical and isotopic evidence so far favors formation of mafic granophyre by local assimilation of mafic country rock—most likely epidiorite—by a single intrusive impact melt phase, which is represented by the regionally homogeneous felsic granophyre.

ABSTRACT The Morrison-Golden Fossil Areas National Natural Landmark, Colorado, USA, including Dinosaur Ridge, is rich in geological and paleontological history, ranking historically as the premier location and type area for Late Jurassic dinosaurs like Stegosaurus and Diplodocus . As the type area for the Morrison Formation, it famously became central in the “Bone Wars” underway in the 1870s. After a brief historical introduction at Stop 1, the trip will explore the ‘mid’-Cretaceous Dakota Sandstone (Stop 2), which yields the top-ranked dinosaur tracksite in the United States, with two type ichnospecies and the most accessible “nest scrape display trace” evidence of dinosaurian sexual display found anywhere. It also an important location for the study of microbial mat in association with dinosaur tracks, now photogrammetrically surveyed in detail. Dinosaur Ridge serves as the type area for the “Dinosaur Freeway” and the transgression of the Western Interior Seaway. Its diverse invertebrate traces have also been described in detail. After lunch at the Dinosaur Ridge Visitor’s Center exhibit (Stop 3), the trip will move a short distance to Golden to view the type localities for the first bird and crocodilian tracks ever reported from the Mesozoic (Stop 4). We will also visit the younger (Late Cretaceous) Laramie Formation, exposed in the Golden clay pits, cut by the Golden fault, and the source of historically famous paleofloras, the first known Ceratopsian tracks, and other type traces now developed as Triceratops Trail (Stop 5), constituting part of the well- documented Fossil Trace and School of Mines Geological Trail complex. The field excursion will involve easy walks and no strenuous climbs.