ABSTRACT Paleocene Lower Wilcox Group sedimentation rates are three times the Cenozoic average for the Gulf of Mexico region and are attributed to Laramide tectonism within the Laramide–Rocky Mountains region. These increased rates likely represent the erosion of easily weathered Phanerozoic strata that blanketed the Laramide-age basement-cored uplifts. Geologic observations and U-Pb geochronology are not sufficient to fully address this hypothesis alone, so we conducted 439 Lu-Hf isotopic analyses on detrital zircons from eight samples from the San Juan Basin and five samples from the Gulf of Mexico Basin. Focusing on the zircons younger than 300 Ma allowed us to make direct comparisons to the eight principal components that comprise the North American Cordilleran magmatic arc: (1) Coast Mountains batholith; (2) North Cascades Range; (3) Idaho batholith; (4) Sierra Nevada batholith; (5) Laramide porphyry copper province; (6) Transverse Ranges; (7) Peninsular Ranges; and (8) Sierra Madre Occidental. The εHf ( t ) results range from +8.9 to –27.0 for the San Juan Basin samples and from +13.0 to –26.6 for the Gulf of Mexico samples. Using the San Juan Basin samples as a proxy for the eroded Mesozoic cover that was shed from the Laramide uplifts, we show that much of the sediment entering the Gulf of Mexico through the Houston and Mississippi embayments during the late Paleocene was derived from reworked cover from the greater Laramide–Rocky Mountains region. However, the Gulf of Mexico samples also include a distinct juvenile suite (εHf [ t ] ranging from +13 to +5) of zircons ranging in age from ca. 220 to 55 Ma that we traced to the Coast Mountains batholith in British Columbia, Canada. This transcontinental connection indicates an extension to the headwaters of the previously defined paleo-Mississippi drainage basin from ca. 58 to 56 Ma. Therefore, we propose a through-going fluvial system (referred to here as the “Coast Mountains River”) that was routed from the Coast Mountains batholith to the Gulf of Mexico. This expands the previously defined paleo-Mississippi drainage basin area by an estimated 280,000 km 2 . Our comprehensive Hf isotopic compilation of the North American Cordilleran magmatic arc also provides a benchmark εHf ( t ) versus U-Pb age plot, which can be used to determine provenance of detrital zircons (85–50 Ma) at the scale of specific region(s) within the Cordillera based on their εHf ( t ) values.

Improved depositional age constraints and stratigraphic description of rocks in San Diego require designation of a new Upper Jurassic formation, herein named the Peñasquitos Formation after its exposures in Los Peñasquitos Canyon Preserve of the city of San Diego. The strata are dark-gray mudstone with interbedded first-cycle volcanogenic sandstone and conglomerate-breccia and contain the Tithonian marine pelecypod Buchia piochii. Laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon 206* Pb/ 238 U ages of 147.9 ± 3.2 Ma, 145.6 ± 5.3 Ma, and 144.5 ± 3.0 Ma measured on volcaniclastic samples from Los Peñasquitos and Rancho Valencia Canyons are interpreted as magmatic crystallization ages and are consistent with the Tithonian depositional age indicated by fossils. Whole-rock geochemistry is consistent with an island-arc volcanic source for most of the rocks. The strata of the Peñasquitos Formation have been assigned to the Santiago Peak volcanics by many workers, but there are major differences. The Peñasquitos Formation is marine; older (150–141 Ma); deformed everywhere and overturned in places; and locally is altered to pyrophyllite. In contrast, the Santiago Peak volcanics are nonmarine and contain paleosols in places; younger (128–110 Ma); undeformed and nearly flat lying in many places; and not altered to pyrophyllite. The Peñasquitos Formation rocks have also been assigned to the Bedford Canyon Formation by previous workers, but the Bedford Canyon is distinctly less volcanogenic and contains chert, pebbly mudstones, and limestone olistoliths(?) with Bajocian- to Callovian-age fossils. Here, we interpret the Peñasquitos Formation as deep-water marine forearc basin sedimentary and volcanic strata deposited outboard of the Peninsular Ranges magmatic arc. The Upper Jurassic Mariposa Formation of the western Sierra Nevada Foothills is a good analog. Results of detrital zircon U/Pb dating from an exposure of continentally derived sandstone at Lusardi Creek are consistent with a mixed volcanic-continental provenance for the Peñasquitos Formation. A weighted mean U/Pb age of 144.9 ± 2.8 Ma from the youngest cluster of detrital grain ages is interpreted as the likely depositional age. Pre-Cordilleran arc zircon age distributions (>285 Ma) are similar to Jurassic deposits from the Colorado Plateau, with dominant Appalachian-derived Paleozoic (300–480 Ma), Pan African (531–641 Ma), and Grenville (950–1335 Ma) grains, consistent with derivation either directly, or through sediment recycling, from the Colorado Plateau Mesozoic basins and related fluvial transport systems. Appalachian- and Ouachita-like detrital zircon age distributions are characteristic of Jurassic Cordilleran forearc basins from northeast Oregon to west-central Baja California, indicating deposition within the same continent-fringing west-facing arc system.

Abstract The Barrick Golden Sunlight Mine (GSM) in Whitehall, Montana, is an industry leader in safe, responsible resource extraction. With more than 3 million ounces of gold poured since 1983, and current proven and probable reserves of 318,000 ounces of gold, GSM is the largest gold producer in Montana. The gold-silver deposit is localized in a hydrothermal breccia pipe related to Late Cretaceous latite porphyry magmatism hosted by the Mesoproterozoic Belt Supergroup, and is influenced by younger cross-cutting faults and fracture systems. The deposit has been mined by both underground and open pit methods, and the current open pit operation was recently permitted for expansion. The mill and tailings operations practice efficient and environmentally responsible resource recovery by processing ore from historical tailings and dumps from around the state in addition to ore from the Golden Sunlight property. This trip will explore the complex geologic and tectonic controls on mineralization and review how GSM has addressed the technical challenges of mining, milling, and reclamation.