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.

Nodular calcareous paleosols are common in the upper member of the Upper Triassic Dolores Formation in the San Juan Mountains of southwestern Colorado. These soils are developed in reddish brown, very fine-grained sandstone and siltstone of a sand-sheet facies that was deposited by eolian and aqueous processes on the margins of a large Triassic erg. Characteristics of these paleosols include nearly complete destruction of physical sedimentary structures, extensive mottling associated with burrows and root trace fossils, poorly sorted textures, and abundant carbonate nodules. Vegetative stabilization of the sand sheet is recorded by trace fossils of long, monopodial root systems, and fine networks of rootlets. Distinctive purple pigmentation of the large root mottles appears to have been produced by more coarsely crystalline hematite, which precipitated in the presence of root-derived organic compounds. Faunal bioturbation in these soils takes the form of meniscate and structureless burrows of the Scoyenia ichnofacies. The meniscate burrows are common in recent soils and pre-Holocene paleosols, and probably represent sediment reworking by arthropods. Carbonate nodules in these soils are composed of micrite and microspar, and they contain sparry calcite crystallaria and septaria. These glaebules occur as individual “floating” entities and as stacked columns. Burrows cross-cut some nodules, indicating that at least some of the pedogenic carbonate accumulations were relatively unlithified at the time of deposition.

The Kirtland Shale or Fruitland Formation directly underlies the Cretaceous-Tertiary boundary throughout most of the San Juan Basin of northwest New Mexico and southwest Colorado. These formations have been known to be Late Cretaceous in age since the early 1900s. Now, with the greatly renewed interest in rocks adjacent to mass extinction boundaries, it is important to place more precise ages on such rock units as the Fruitland and Kirtland. Deposition of the Fruitland and Kirtland was closely related to deposition of the underlying marine-regressive Pictured Cliffs Sandstone. Because the Pictured Cliffs was deposited as a strandline sandstone in a subsiding seaway, its stratigraphic expression, when related to a time horizon (the Huerfanito Bentonite Bed), is a series of rising-to-the-northeast, time transgressive, stair steps. Thus, time lines (or horizons) drawn parallel to the Huerfanito cut through the marine Lewis Shale, the strandline Pictured Cliffs Sandstone, and the continental Fruitland Formation and Kirtland Shale. Ammonites have been collected and identified from various stratigraphic levels within the Lewis Shale around the northwest, north, and east sides of the San Juan Basin. These fossils can be tied in to the established ammonite zonation of the Western Interior seaway. Because some of these ammonite zones have been radiometrically dated outside the San Juan Basin, it is possible to project these dated faunal zones from the Lewis Shale along time lines into the Fruitland Formation and Kirtland Shale and thereby estimate the age of those rocks. Based on these projections the part of the Fruitland and Kirtland laterally time-equivalent to the Lewis Shale is estimated to range from 73.2 ± 0.7 Ma to 73.9 ± 0.8 Ma. The average age for this interval based on these dates is 73.5 ± 0.5 Ma; the maximum range of the interval at the 95 percent confidence level is 71.8 to 75.5 Ma. This age range puts these rocks in the Campanian Stage of the upper Cretaceous in the San Juan Basin.

Lithofacies analysis of the Tertiary Ojo Alamo Sandstone and related strata in the San Juan Basin indicates that Laramide (Late Cretaceous–early Tertiary) volcanism and uplift north of the present-day San Juan Basin controlled sedimentation patterns of Upper Cretaceous and lower Tertiary rocks. Eight major lithofacies reflect changes in sedimentation that occurred during this time. The Ojo Alamo Sandstone is characterized in most areas of the San Juan Basin by a pebbly, trough-crossbedded lithofacies. A related channel-form sandstone and shale facies makes up the Ojo Alamo at Mesa Portales. Both lithofacies include both sediment derived from north of the present-day San Juan Basin and sediment eroded and reworked from (1) a carbonaceous shale and channel-form sandstone facies, (2) a shale and volcaniclastic sandstone facies, and (3) a volcaniclastic conglomerate and sandstone facies. The pebbly, trough-crossbedded lithofacies, which was deposited by streams on alluvial plains, differs in grain size, pebble composition, and transport direction on the east and west sides of the present-day basin. At least two distinct source areas for the streams are suggested by these differences. One source is in the area of the present-day Needle Mountains and western San Juan Mountains. A second source is located in the area of the central to eastern San Juan Mountains of southwest Colorado. Sediments deposited by alluvial streams in the western San Juan basin include sand- and pebble-size material. Initially, Ojo Alamo streams carried up to 25 percent volcanic pebbles reworked from the Animas Formation or from Upper Cretaceous andesitic flows in the source area. Later streams, however, carried an increasing percentage of quartz pebbles over volcanic pebbles. Lithofacies of the Ojo Alamo in the eastern San Juan Basin include channel sandstone and conglomerates and a channel-form sandstone and shale facies. Compared to sediments of the western alluvial complex, the eastern sediments (mapped as Ojo Alamo Sandstone, upper part of the Animas Formation, and Nacimiento Formation) are finer grained, contain few pebbles, contain less than 1 percent volcanic pebbles, and show different transport directions. Mudstone interbeds are thicker and more abundant, especially at Mesa Portales where an accompanying down-dip change in the alluvial system contributes to formation of the channel-form sandstone and shale lithofacies.