Russell F. Dubiel

The Pangaean megamonsoon - evidence from the Upper Triassic Chinle Formation, Colorado Plateau

The Upper Triassic Chinle Formation was deposited at an exceptional time in Earths paleogeographic and paleoclimatic history. During the Triassic, the supercontinent Pangaea was at its greatest size, in terms of both aggregated continental crust and exposed land area. Moreover, the exposed land was divided symmetrically about the paleoequator between the northern and southern hemispheres. These conditions were ideal for maximizing monsoonal circulation, as predicted from paleoclimate models. The Chinle was deposited between about 5?to 15?N paleolatitude in the western equatorial region of Pangaea, a key area for documenting the effects of the monsoonal climate. This study summarizes sedimentologic and paleontologic data from the Chinle Formation on the Colorado Plateau and integrates that data with paleoclimatic models. The evidence for abundant moisture and seasonality attest to the reversal of equatorial flow and support the hypothesis that the Triassic Pangaean climate was dominated by monsoonal circulation. The Chinle Formation contains continental lithofacies deposited in fluvial channels, crevasse splays, lakes, bogs, marshes, and lacustrine deltas that reflect abundant precipitation and shallow water tables. Paleosols and ichnofossils indicate that water tables and lake levels fluctuated episodically. Interbedded lacustrine carbonates and marginal-lacustrine siltstones and mudstones indicate longer-term but regular, episodic fluctuations in lake level. Fine-scale laminations in lacustrine carbonates suggest a seasonal influx of clastic sediment, and thus precipitation, to the basin. Uppermost Chinle strata consist of lacustrine and marginal-lacustrine mudstones interbedded with minor eolian sand sheets and eolian dunes; thus, the later Triassic reflects continued precipitation, but was marked by more pronounced and extended dry seasons.

Application of morphologic burrow interpretations to discern continental burrow architects: Lungfish or crayfish?

A methodology for trace fossil identification using burrowing signatures is tested by evaluating ancient and modern lungfish and crayfish burrows and comparing them to previously undescribed burrows in a stratigraphic interval thought to contain both lungfish and crayfish burrows. Permian burrows that bear skeletal remains of the lungfish Gnathorhiza, from museum collections, were evaluated to identify unique burrow morphologies that could be used to distinguish lungfish from crayfish burrows when fossil remains are absent. The lungfish burrows were evaluated for details of the burrowing mechanism preserved in the burrow morphologies together forming burrowing signatures and were compared to new burrows in the Chinle Formation of western Colorado to test the methodology of using burrow signatures to identify unknown burrows. Permian lungfish aestivation burrows show simple, nearly vertical, unbranched architectures and relatively smooth surficial morphologies with characteristic quasi‐horizontal striae on...

Plant taphonomy in incised valleys: Implications for interpreting paleoclimate from fossil plants

Paleoclimatic interpretations of the Upper Triassic Chinle Formation (Colorado Plateau) based on plants conflict with those based on the sedimentary rocks. The plants are suggestive of a humid, equable climate, whereas the rocks are more consistent with deposition under highly seasonal precipitation and ground-water conditions. Fossil plant assemblages are limited to the lower members of the Chinle Formation, which were deposited within incised valleys that were cut into underlying Lower to Middle Triassic and older rocks. In contrast, the upper members of the formation, which were deposited across the fluvial plain after the incised valleys were filled, have few preserved fossil plants. The taphonomic characteristics of the plant fossil assemblages, within the stratigraphic and hydrologic context of the incised valley-fill sequence, explain the vertical and lateral distribution of these assemblages. The depositional, hydrological, and near-surface geochemical conditions were more conducive to preservation of the plants. Fossil plant assemblages in fully terrestrial incised-valley fills should be taphonomically biased toward riparian wetland environments. If those assemblages are used to interpret paleoclimate, the paleoclimatic interpretations will also be biased. The bias may be particularly strong in climates such as those during deposition of the Chinle Formation, when the riparian wetlands may reflect local hydrologic conditions rather than regional climate, and should be taken into account when using these types of plant assemblages in paleoclimatic interpretations.

Termite (Insecta: Isoptera) nest ichnofossils from the upper triassic chinle formation, petrified forest national park, Arizona

A fossil nest (calie), including chambers and galleries (runaways), from the lower part of the Petrified Forest Member of the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, is similar to nests constructed by modern primitive termites (Insecta: Isoptera), which reflect cooperation in nest construction typical of complex social behavior. A new ichnogenus and ichno‐species, Archeoentomichnus metapolypholeos, is proposed for the distinctive ichnofossil that may represent the efforts of a social caste system in primitive termites. The Late Triassic (Late Carnian) nest also may represent the earliest known fossil evidence of Isoptera (termites) and is suggestive of the antiquity of social behavior among insects. Social behavior in insects, including termites, was previously thought to have evolved in the Early Cretaceous in conjunction with the evolutionary radiation of angiosperms. Because the fossil and ichnofossil record of insects and organisms suggested to be insects dates back...

Eocene extension of early Eocene lacustrine strata in a complexly deformed Sevier-Laramide hinterland, northwest Utah and northeast Nevada

New data from a small enclave that did not undergo significant Neogene tectonism in northwest Utah document Eocene tilting, normal faulting, land folding of the early Eocene lacustrine White Sage Formation. This deformation was part of a complexly deformed Eocene Sevier-Laramide hinterland that underwent east-west contraction and extension in different areas and ultimately gave way to widespread Oligocene extension in the eastern Great Basin. Such “mixed-mode” supracrustal Eocene tectonism may reflect local adjustments to maintain a state of regional gravitational equilibrium, or may record responses to changing plate convergence rates at the west edge of the continent.

Hands-On Geology for Navajo Nation Teachers

The Navajo Nation comprises the largest land area and the largest population of any Native American community in the United States, and it hosts some of Earths most spectacular geology. Geologic resources and environmental concerns have figured prominently in the Navajo Nations recent history and economy. Despite the traditional cultural ties of the Dine (Navajo) people to their land, and the natural curiosity of most K-12 students about the earth, geoscience courses are virtually absent from curricula in Navajo Nation public schools. Many native-born teachers are familiar with the landscape and its cultural geography, but few have received formal training in geology and are not comfortable introducing it into their classrooms. A collaborative project between the U.S. Geological Survey and Navajo Community College has addressed this issue on a local scale by developing a classroom and field- intensive one-week summer course for Navajo Nation teachers with no previous geologic experience. The course intr...

Architectural-Fades Analysis of Nonmarine Depositional Systems in the Upper Triassic Chinle Formation, Southeastern Utah

The Upper Triassic Chinle Formation in southeastern Utah consists of nonmarine strata deposited in a back-arc cratonic basin. Architectural analysis of exposures in subparallel canyons reveals an intricately interbedded fluvial-deltaic-lacustrine system characterized by (1) mobile fluvial-channel belts consisting of stacked channel complexes; (2) overbank deposits of levees, paleosols, marshes, and small flood-plain lakes interfingered with crevasse splays and lacustrine deltas; and (3) extensive lacustrine strata deposited over the entire study area. Stratigraphic panels using closely spaced measured sections drawn either perpendicular or parallel to depositional dip show lateral facies relations within the extra-channel strata and the relations to channel complexes. Successions of panels through several canyons depict the three-dimensional architecture of the Chinle depositional system.

Assessment of undiscovered oil and gas resources in the Eagle Ford Group and associated Cenomanian–Turonian Strata, U.S. Gulf Coast, Texas, 2018

The U.S. Geological Survey (USGS) assessed undiscovered, technically recoverable hydrocarbon resources in self-sourced continuous reservoirs of the Upper Cretaceous Eagle Ford Group and associated Cenomanian–Turonian strata, which are present in the subsurface across the U.S. Gulf Coast region, Texas (fig. 1). The USGS completes geologybased assessments using the elements of the total petroleum system (TPS), which include source rock thickness, organic richness, and thermal maturity for self-sourced continuous accumulations. Assessment units (AUs) within a TPS are defined by strata that share similar structural and petroleum-charge histories along with lithology and stratigraphy.

Assessment of undiscovered oil and gas resources in sandstone reservoirs of the Cotton Valley Group, U.S. Gulf Coast, 2015

Using a geology-based assessment methodology, the U.S. Geological Survey estimated undiscovered mean volumes of 14 million barrels of conventional oil, 430 billion cubic feet of conventional gas, 34,028 billion cubic feet of continuous gas, and a mean total of 391 million barrels of natural gas liquids in sandstone reservoirs of the Upper Jurassic–Lower Cretaceous Cotton Valley Group in onshore lands and State waters of the U.S. Gulf Coast region.

Paleoclimate cycles and tectonic controls on fluvial, lacustrine, and eolian strata in upper Triassic Chinle Formation, San Juan basin

Sedimentologic study of the Upper Triassic Chinle Formation in the San Juan basin (SJB) indicates that Late Triassic paleoclimate and tectonic movements influenced the distribution of continental lithofacies. The Shinarump, Monitor Butte, and Petrified Forest Members in the lower part of the Chinle consist of complexly interfingered fluvial, floodplain, marsh, and lacustrine rocks; the Owl Rock and Rock Point Members in the upper part consists of lacustrine-basin and eolian sandsheet strata. Facies analysis, vertebrate and invertebrate paleontology, and paleoclimate models demonstrate that the Late Triassic was dominated by tropical monsoonal circulation, which provided abundant precipitation interspersed with seasonally dry periods. Owl Rock lacustrine strata comprise laminated limestones that reflect seasonal monsoonal precipitation and larger scale, interbedded carbonates and fine-grained clastics that represent longer term, alternating wet and dry climatic cycles. Overlying Rock Point eolian sand-sheet and dune deposits indicate persistent alternating but drier climatic cyclicity. Within the Chinle, upward succession of lacustrine, alternating lacustrine/eolian sand-sheet, and eolian sand-sheet/dune deposits reflects an overall decrease in precipitation due to the northward migration of Pangaea out of low latitudes dominated by monsoonal circulation.