ABSTRACT The most common and widespread sedimentary facies of Pleistocene Lake Bonneville, in the eastern Great Basin of North America, is marl, which consists of a mixture of fine-grained endogenic calcium carbonate that precipitated in the epilimnion of the lake and then settled onto the lake floor and mixed with fine-grained clastic sediments. Primary sources of clastic sediment were inflowing rivers, wave activity in shore zones, and ice rafting. The thickness of deposits in cores and outcrops is largely dependent on the proportion of clastic sediment, although the rate of endogenic calcium carbonate precipitation probably also varied temporally and spatially. Net sediment-accumulation rate in the marl, as measured in outcrops and cores, ranges from a low of 4 cm/1000 yr, in the middle of the lake basin far from sources of clastic input, to over 100 cm/1000 yr near clastic-sediment sources. Underflow deposits, derived from higher-density river water loaded with suspended sediment, are thick and extensive near the mouths of major rivers that drained glaciated mountains. Net sediment-accumulation rates in suspended-load underflow deposits were much greater than those in contemporaneously deposited marl. The largest underflow-sediment accumulations, which have a fan shape in plan view, have been referred to as deltas (as at the mouths of the Sevier, Provo, Weber, and Bear Rivers). True Gilbert-type deltas composed of gravel, with topset, foreset, and bottomset beds, are uncommon in the basin. Variability in the sedimentary characteristics of the Bonneville deposits is determined by geomorphic factors, such as wave energy, composition of surficial material in the shore zone (e.g., resistant bedrock vs. unconsolidated alluvium), slope, and proximity to river mouths and active shore zones.

Abstract Carlin-type gold deposits in Nevada account for ~5% of worldwide annual gold production, typically about ~135 metric tons (t) (~4.5 Moz) per year. They are hydrothermal epigenetic replacement bodies hosted predominantly in carbonate-bearing sedimentary rocks. They are known for their “invisible” gold that occurs in the crystal structure of pyrite. Over 95% of the production from these deposits is from four clusters of deposits, which include the Carlin trend and the Cortez, Getchell, and Jerritt Canyon camps. Despite differences in the local geologic settings, the characteristics of the deposits are very similar in the four clusters. Shared characteristics include: (1) alteration characterized by carbonate dissolution, silicate argillization, and silicification; (2) ore formation characterized by auriferous arsensian pyrite, typically as rims on preore pyrite, followed by late open-space deposition of orpiment, realgar, stibnite, and other minerals; (3) Ag/Au ratios of <1 in ore; (4) an As-Hg-Sb-Tl geochemical signature; (5) low temperatures (~160°–240°C) and salinities of ore fluids (~1–6 wt % NaCl equiv) and fairly shallow depths of formation (<~2–3 km); and (6) lack of mineral and elemental zoning around ore. The four clusters share regional geologic controls related to formation as follows: (1) along the rifted margin of a craton, (2) within the slope facies of a passive margin sequence dominated by carbonates, (3) in the lower plate of a regional thrust fault, and (4) during a narrow time interval in the late Eocene (~42–34 Ma). The geometries and ore controls of the deposits in the four clusters are also very similar. At the deposit scale, ore and hydrothermal alteration are commonly associated with high-angle faults and preore low-angle contractional structures, including thrust faults and folds. The high-angle faults acted as fluid pathways for upwelling ore fluids, which were then diverted into lower angle favorable strata and contractional structures, where fluid-rock interaction led to replacement of carbonate and formation of ore. Rheologic contrasts between lithologies were also critical in diverting fluids into wall rocks. Common rheologic contrasts include contacts between thin- and thick-bedded lithologic units and the margins of contact metamorphic aureoles associated with Mesozoic intrusions. The similarities suggest common processes. Four critical processes are apparent: (1) development of source(s) for gold and other critical components of the ore fluids, (2) formation of fluid pathways, (3) water-rock interaction and gold deposition, and (4) a tectonic trigger, which was renewal of magmatism and a change from contraction to extension in the late Eocene. Consensus exists on these processes, except for the source of gold and other components of the ore fluid, with most models calling upon either a magmatic-hydrothermal source or a crustal source, where metals were scavenged by either meteoric or metamorphic fluids. Future research should focus on Carlin-style deposits in Nevada that exhibit epithermal characteristics and deposits that appear to have a clear genetic association with magmatic-hydrothermal systems associated with upper crustal intrusions. Rather than discrete types of ore deposits, there may be continua between Carlin-type gold deposits, epithermal deposits, and distal disseminated deposits, with the four large camps representing an end member.

Abstract On this field trip, you will visit two important archaeological cave sites that provide the most compelling evidence for latest Pleistocene and earliest Holocene human occupation in the Bonneville Basin. Danger Cave, located near Wendover, Utah/Nevada, is famed for its deeply stratified archaeological deposits dating as old as 10,300 radiocarbon yr B.P., when the remnant of Lake Bonneville stood at the Gilbert shoreline. Bonneville Estates Rockshelter, located south of Danger Cave at the Lake Bonneville highstand shoreline, also contains well-preserved stratified deposits, including artifacts and cultural features dated to at least 11,000 radiocarbon yr B.P., making it one of the oldest known archaeological occupations in the Great Basin. We describe results of our recent research at these sites and show the stratigraphic evidence for these earliest human occupations. We also review recent work at the Old River Bed Delta, on Dugway Proving Ground, that has documented hundreds of Paleoarchaic occupation sites dating 11,000–8500 radiocarbon yr B.P. Together these localities give us an unparalleled picture of human occupation during the first few thousand years of known human occupation in the region, during a time of dramatic environmental change. Packrat middens, pollen sampling localities, and geomorphic features that illustrate the history of Pleistocene Lake Bonneville and the environmental history of the western Bonneville Basin will also be observed on this trip .

Abstract Donald R. Currey spent over two decades researching and exploring relics of ancient Lake Bonneville in the eastern Great Basin. Shoreline and deepwater deposits of Lake Bonneville document coastal processes, lake chemistry, and environmental change during the late Pleistocene and Holocene. This field guide summarizes findings at many of the classic localities researched by Currey and colleagues that contributed to the current understanding of this impressive pluvial lake and its interglacial successor, Great Salt Lake. Subjects include coastal processes at Antelope Island and the Stockton Bar; lake history, chemistry and environmental change at Stansbury Island, the Public Shooting Grounds and Hansel Valley; deltaic depositional processes at Big Cottonwood Canyon, American Fork Canyon and Brigham City; and the relative chronology of glacial and lacustrine deposition at Little Cottonwood Canyon and Bells Canyon.