Tectonic and eustatic control of Mesaverde Group (Campanian−Maastrichtian) architecture, Wyoming-Utah-Colorado region, USA

We describe and analyze the depositional history and stratigraphic architecture of the Campanian and Maastrichtian succession of the southern greater Green River basin of Wyoming, USA, and surrounding areas to better understand the interplay between tectonic and eustatic drivers that built the stratigraphy. By integrating new measured sections with published outcrop, well-log, and paleogeographic data, two new stratigraphic correlation diagrams, 35 new paleogeographic reconstructions, and six new tectonic diagrams were created for this part of the Western Interior Seaway. From this work, two time-scales of organization are evident: (1) 100−300 k.y.-scale, mainly eustatically driven regressive-transgressive shoreline oscillations that generated repeated sequences of alluvial-coastal plain-shoreline deposits, passing basinward to subaqueous deltas, then capped by transgressive estuarine/barrier lagoon deposits, and (2) 3.0−4.0 m.y.-scale, tectonically driven groups of 10 to 15 of these eustatically driven units stacked in an offset arrangement to form larger clastic units, which are herein referred to as clastic wedges. Four regional clastic wedges are recognized, based on the architectures of these clastic packages. These are the: (1) Adaville, (2) Rock Springs, (3) Iles, and (4) Williams Fork clastic wedges. Pre-Mesaverde deposition in the Wyoming-Utah-Colorado (USA) region during the Middle Cretaceous was characterized by thickening of the clastic wedge close to the thrust-front, driven primarily by retroarc foreland basin (flexural) tectonics. However, a basinward shift in deposition during the Santonian into the early Campanian (Adaville clastic wedge) signaled a change in the dominant stratigraphic drivers in the region. Shoreline advance accelerated in the early to middle Campanian (Rock Springs clastic wedge), as the end of activity in the thrust belt, growing importance of flat-slab subduction, and steady eastward migration of the zone of dynamic subsidence led to loss of the foredeep and forebulge, with the attendant formation of a low-accommodation shelf environment. This “flat-shelf” environment promoted large shoreline advances and retreats during sea-level rise and fall. During the middle to late Campanian (Iles clastic wedge), deep erosion on the crest of the Moxa Arch, thinning on the crests of the Rock Springs and Rawlins uplifts, and subsequent Laramide-driven basin formation occurred as the Laramide blocks began to partition the region. The next clastic package (Williams Fork clastic wedge) pushed the shoreline over 400 km away from the thrust belt during the late Campanian. This was followed by a very large and persistent marine transgression across the region, with the formation of a Laramide-driven deepwater turbidite basin with toe-of-slope fans into the early Maastrichtian. The Mesaverde Group in the Wyoming-Utah-Colorado region is thus characterized by: (1) a succession of four tectonically driven classic wedges, each comprised of a dozen or so eustatically driven packages that preserve large basinward and landward shoreline shifts, (2) broad regional sand and silt dispersal on a low-accommodation marine shelf setting, (3) a progressive, tectonically driven, basinward shift of deposition with offset, basinward stacking of successive clastic wedges, and (4) the gradual formation of various uplifts and sub-basins, the timing and sizes of which were controlled by the movement of deep-seated Laramide blocks. The Mesaverde Group in the Wyoming-Utah-Colorado region provides an outstanding opportunity to study the dynamic interaction among the tectonic control elements of a subducting plate (crustal loading-flexure, dynamic subsidence/uplift, and regional flat-slab basin partitioning), as well as the dynamic interaction of tectonic and eustatic controls.

Initial Laramide tectonism recorded by Upper Cretaceous paleoseismites in the northern Bighorn Basin, USA: Field indicators of an applied end load stress

Soft-sediment deformational structures associated with paleoseismicity (e.g., planar clastic dikes) exist within Upper Cretaceous Mesaverde Group strata in the Laramide Elk Basin anticline, northern Bighorn Basin (Wyoming, USA). Retrodeformation of the Elk Basin anticline to a horizontal Mesaverde Group position indicates that all basement offset is removed and that clastic dikes exhibit a dominant northeast trend. The trend of clastic dikes corresponds to the interpreted northeast-southwest direction of early Laramide layer-parallel shortening, suggesting that the development of clastic dikes recorded initiation of basement deformation and Laramide tectonism. To determine the timing of clastic dike development, we present zircon U-Pb geochronology from the stratigraphically lowest sand-source bed generating upwardly injected clastic dikes and a volcanic bentonite bed (Ardmore bentonite) above the stratigraphic interval containing clastic dikes. Weighted mean ages bracket clastic dike development between 82.4 and 78.0 Ma. Our results imply initiation of basement deformation ∼8–15 m.y. prior than other estimates in the Bighorn Basin. Therefore, we interpret the development of clastic dikes in the Elk Basin anticline to represent an initial phase of Laramide tectonism associated with an applied end load stress transmitted from the southwestern North American plate margin in response to the collision of the conjugate Shatsky Rise oceanic plateau ca. 90–85 Ma. Results demonstrate how sedimentary responses in the foreland can be used to understand tectonic processes at plate boundaries and provide spatial-temporal parameters for models of Laramide deformation.

On the occurrence of fossil conifers with affinities to Geinitzia in the late Cretaceous (Campanian) Mesaverde Group, Williams Fork Formation of northeastern Utah, U.S.A.

In this paper we describe fossil conifer branches discovered in the Mesaverde Group, Williams Fork Formation in northeastern Utah, along Snake John Reef. Fossil conifers from the Campanian of northeastern Utah have not been previously studied, despite their common occurrence in the formation. The recovered fossils closely resemble Geinitzia known from the late Cretaceous of Europe, with several previous reported occurrences in North America, including New Jersey and Southern Utah. The fossils share morphological characteristics with Geinitzia, exhibiting short spirally arranged thin needles, with appressed scale-like leaves along the shoots. They differ from Araucarites in that the appressed needles are more scale like and smaller, and differ from the members of the modern Araucariaceae in lacking broad bases to the needles, although the fossils resemble the modern species Araucaria heterophylla (Norfolk Island Pine) native to the South Pacific. The observed branching pattern in the fossil reflects similarities found in the Cupressaceae Family, and Geinitzia may be regarded as an early member of this group, or having an affinity to fossil Araucariaceae conifers, which despite having a modern southern hemisphere geographic distribution today were widespread during the late Mesozoic, extending across North America and Europe.

On the occurrence of fossil conifers with affinities to Geinitzia in the late Cretaceous (Campanian) Mesaverde Group, Williams Fork Formation of northeastern Utah, U.S.A.

In this paper we describe fossil conifer branches discovered in the Mesaverde Group, Williams Fork Formation in northeastern Utah, along Snake John Reef. Fossil conifers from the Campanian of northeastern Utah have not been previously studied, despite their common occurrence in the formation. The recovered fossils closely resemble Geinitzia known from the late Cretaceous of Europe, with several previous reported occurrences in North America, including New Jersey and Southern Utah. The fossils share morphological characteristics with Geinitzia, exhibiting short spirally arranged thin needles, with appressed scale-like leaves along the shoots. They differ from Araucarites in that the appressed needles are more scale like and smaller, and differ from the members of the modern Araucariaceae in lacking broad bases to the needles, although the fossils resemble the modern species Araucaria heterophylla (Norfolk Island Pine) native to the South Pacific. The observed branching pattern in the fossil reflects similarities found in the Cupressaceae Family, and Geinitzia may be regarded as an early member of this group, or having an affinity to fossil Araucariaceae conifers, which despite having a modern southern hemisphere geographic distribution today were widespread during the late Mesozoic, extending across North America and Europe.