Geology topics

Mark W. Longman

Publications and source records attributed to Mark W. Longman.

2 recordsLinked to original sources

USGS2023

The edge of a Permian erg: Eolian facies and provenance of the Lyons Sandstone in northern Colorado

Facies of the Permian Lyons Sandstone are described and interpreted based on analyses of 23 cores from Larimer and Weld counties, Colorado. Here, the Lyons Sandstone consists of very fine- to medium-grained sandstone with minor silt and mudstone interbeds. The unit has five recurrent siliciclastic facies that can be grouped into two facies associations (FA). FA1 consists of (1) high-angle, cross-laminated sandstone (Facies 1; interpreted as eolian dune remnants); (2) low-angle, cross-laminated and horizontally laminated sandstone (Facies 2; interdune); and (3) chaotically bedded to folded sandstone (Facies 3; lower dune flanks). FA2 , in contrast, is mainly (4) wavy- to irregularly laminated silty sandstone (Facies 4; wet to damp interdune); and (5) massive to wavy-laminated silt-rich mudstone (Facies 5; ponded water areas between dunes) with minor amounts of high-angle, cross-laminated sandstone (Facies 1) and low-angle, cross-laminated and horizontally laminated sandstone (Facies 2). FA1 is hypothesized to have been produced in an eolian system akin to those that might exist in the dune-dominated portion of an erg, whereas FA2 was deposited in the intermittently wet portion of this eolian system, perhaps along erg margins or in flat dune-adjacent settings that were impacted by the water table. Isopach data suggests that the study area is on the fringe of a larger Lyons system that spans > 100,000 km 2 , and was deposited close to the Ancestral Rockies—a paleogeography consistent with deposition in erg to erg-margin paleoenvironments. Detrital zircon populations from nearby Colorado Front Range outcrops and from 12 correlative eolian units are dominated by small, well-rounded Paleoproterozoic and Mesoproterozoic grain populations that are remarkably similar between units, signaling a well-mixed system that also received an influx of distally sourced sediment from the Appalachian orogen. Detrital zircon-based maximum depositional ages of the Lyons Sandstone and its equivalents are internally consistent with deposition of the unit during the latest Artinskian to Kungurian.

Colorado↗

USGS2022

Provenance of Devonian-Carboniferous strata of Colorado: The influence of the Cambrian and the Proterozoic

We report new LA-ICPMS U-Pb detrital zircon ages and sedimentary petrology of silty to sandy limestones and dolostones, as well as calcareous to dolomitic sandstones of the Devonian-Carboniferous (Mississippian) Chaffee Group, as well as detrital zircon ages from the Late Cambrian Sawatch Quartzite and a U-Pb zircon crystallization age on a late Mesoproterozoic (1087.9 13.5 Ma) granitoid of underlying basement from the Eagle basin of Colorado. Grain populations in the Chaffee Group are mostly bimodal, with over 84% of zircons centered around a Paleoproterozoic (ca. 1.78 Ga) mode typical of the Yavapai Province that forms much of the basement of Colorado and an early Mesoproterozoic (ca. 1.42 Ga) mode typical of A-type granites that intrude this region. A notable late Mesoproterozoic (ca. 1.08 Ga) mode exists in some Chaffee samples, giving those samples a trimodal detrital zircon age distribution. These bipartite or tripartite detrital zircon age modes exist in Cambrian, Devonian and Carboniferous strata from paleogeographically adjacent successions, but the correlation between the Chaffee zircons is highest with the regions basal Cambrian sandstones (Sawatch, Flathead, Ignacio formations) which have similar (1.08 Ga, 1.43 Ga, 1.71 Ga) zircon populations and a paucity of >1.8 Ga grains. This similarity suggests that the majority of grains in the Chaffee Group derive from recycling of these basal sandstones, and that little sediment was derived directly from then-exposed Precambrian basement highs, from the Wyoming Craton to the north, or from Paleoproterozoic arcs and orogens to the west and northeast. Minor Mesoarchean to early Paleoproterozoic (ca. 3.00 to 2.40 Ga) grains exist in the Chaffee Group, an attribute shared by the Late Ordovician Harding Sandstone of Colorados Front Range, but that is absent from the regions underlying Cambrian sandstonessuggesting some recycled mixture of Cambrian and Ordovician sedimentary rocks. No near-depositional age grains are present in the Chaffee Group; the youngest grain is Early Devonian (~417 Ma), >45 m.y. older than these strata, and Paleozoic grains are extremely uncommon (<0.1%; n=2927 grains).

Colorado↗