Tectonic influence on axial-transverse sediment routing in the Denver Basin

ABSTRACT Detrital zircon U-Pb and (U-Th)/He ages from latest Cretaceous–Eocene strata of the Denver Basin provide novel insights into evolving sediment sourcing, recycling, and dispersal patterns during deposition in an intracontinental foreland basin. In total, 2464 U-Pb and 78 (U-Th)/He analyses of detrital zircons from 21 sandstone samples are presented from outcrop and drill core in the proximal and distal portions of the Denver Basin. Upper Cretaceous samples that predate uplift of the southern Front Range during the Laramide orogeny (Pierre Shale, Fox Hills Sandstone, and Laramie Formation) contain prominent Late Cretaceous (84–77 Ma), Jurassic (169–163 Ma), and Proterozoic (1.69–1.68 Ga) U-Pb ages, along with less abundant Paleozoic through Archean zircon grain ages. These grain ages are consistent with sources in the western U.S. Cordillera, including the Mesozoic Cordilleran magmatic arc and Yavapai-Mazatzal basement, with lesser contributions of Grenville and Appalachian zircon recycled from older sedimentary sequences. Mesozoic zircon (U-Th)/He ages confirm Cordilleran sources and/or recycling from the Sevier orogenic hinterland. Five of the 11 samples from syn-Laramide basin fill (latest Cretaceous–Paleocene D1 Sequence) and all five samples from the overlying Eocene D2 Sequence are dominated by 1.1–1.05 Ga zircon ages that are interpreted to reflect local derivation from the ca. 1.1 Ga Pikes Peak batholith. Corresponding late Mesoproterozoic to early Neoproterozoic zircon (U-Th)/He ages are consistent with local sourcing from the southern Front Range that underwent limited Mesozoic–Cenozoic unroofing. The other six samples from the D1 Sequence yielded detrital zircon U-Pb ages similar to pre-Laramide units, with major U-Pb age peaks at ca. 1.7 and 1.4 Ga but lacking the 1.1 Ga age peak found in the other syn-Laramide samples. One of these samples yielded abundant Mesozoic and Paleozoic (U-Th)/He ages, including prominent Early and Late Cretaceous peaks. We propose that fill of the Denver Basin represents the interplay between locally derived sediment delivered by transverse drainages that emanated from the southern Front Range and a previously unrecognized, possibly extraregional, axial-fluvial system. Transverse alluvial-fluvial fans, preserved in proximal basin fill, record progressive unroofing of southern Front Range basement during D1 and D2 Sequence deposition. Deposits of the upper and lower D1 Sequence across the basin were derived from these fans that emanated from the southern Front Range. However, the finer-grained, middle portion of the D1 Sequence that spans the Cretaceous-Paleogene boundary was deposited by both transverse (proximal basin fill) and axial (distal basin fill) fluvial systems that exhibit contrasting provenance signatures. Although both tectonic and climatic controls likely influenced the stratigraphic development of the Denver Basin, the migration of locally derived fans toward and then away from the thrust front suggests that uplift of the southern Front Range may have peaked at approximately the Cretaceous-Paleogene boundary.

Giant meandering channel evolution, Campos deep-water salt basin, Brazil

Submarine channels are conduits for sediment delivery to continental margins, and channel deposits can be sandy components of the fill in tectonically active salt basins. Examples of salt-withdrawal basin fill commonly show successions of sandy channelized or sheet-like systems alternating with more mud-rich mass-transport complexes and hemipelagites. This alternation of depositional styles is controlled by subsidence and sediment-supply histories. Salt-basin fill comprising successions of largely uninterrupted meandering-channel deposition are less commonly recognized. This begs the questions: can sediment supply be large enough to overwhelm basin subsidence and result in a thick succession of channel deposits, and, if so, how would such a channel system evolve? Here, we use three-dimensional seismic-reflection data from a >1500 km2 region with salt-influenced topography in the Campos Basin, offshore Brazil, to evaluate the influence of salt diapirs on an Upper Cretaceous–Paleogene giant meandering submarine-channel system (channel elements >1 km wide; meander wavelengths several kilometers to >10 km). The large scale of the channels in the Campos Basin suggests that sediment discharge was large enough to sustain the meandering channel system in spite of large variability in subsidence across the region. We interpreted 22 channel centerlines to reconstruct the detailed kinematic evolution of this depositional system; this level of detail is akin to that of recent studies of meandering fluvial channels in time-lapse Landsat satellite images. The oldest channel elements are farther from salt diapirs than many of the younger ones; the centerlines of the older channel elements exhibit a correlation between curvature and migration rate, and a spatial delay between locations of peak curvature and maximum migration distance, similar to that observed in rivers. As many of the younger channel centerlines expanded toward nearby salt diapirs, their migration pattern switched to downstream translation as a result of partial confinement. Channel segments that docked against salt diapirs became less mobile, and, as a result, they do not show a correlation between curvature and migration rate. The channel migration pattern in the Campos Basin is different compared to that of a tectonically quiescent continental rise where meander evolution is unobstructed. This style of channelized basin filling is different from that of many existing examples of salt-withdrawal minibasins that are dominated by overall less-channelized deposits. This difference might be a result of the delivery of voluminous coarse sediment and high discharge of channel-forming turbidity currents to the Campos Basin from rivers draining actively uplifting coastal mountains of southeastern Brazil. Detailed kinematic analysis of such well-preserved channels can be used to reconstruct the impact of structural deformation on basin fill.

Cross-border megasequence stratigraphy of the Northern, Central and Southern North Sea: a comparative tectono-stratigraphic synthesis

The Devonian-Recent tectono-stratigraphic history of the Northern, Central and Southern North Sea is here reviewed at a regional scale and four novel cross-border pseudo-Wheeler diagrams are presented to summarize the stratigraphic evolution of the cycles of basin fill and uplift/erosion. In this scheme, six first-order megasequence boundaries have been defined, characterized by extensive and long-lasting erosional hiatuses and major coastal regressions: (1) Caledonian (or Base Devonian) Unconformity; (2) Variscan-Saalian (or Base Permian) Unconformity; (3) Mid Cimmerian (or Intra-Aalenian) Unconformity; (4) Late Cimmerian (or Base Cretaceous) Unconformity; (5) Atlantean (or Near-Base Tertiary) Unconformity; (6) Eridanos (or Mid-Miocene) Unconformity. These surfaces have been linked to regional causal factors ranging from: orogenesis-related compressional uplifts, in either active plate margin settings (1) or foreland basin settings (2); intra-plate dynamically supported uplifts associated with the development of mantle plumes (3, 5 and 6); the end-of-rifting and associated widespread erosion of tilted fault block crests (4).The aforementioned megasequence boundaries punctuate the geodynamic evolution of the North Sea area and facilitate the sub-division of the entire the North Sea sedimentary basin fill into six megasequences, named here from A to F. All the lithostratigraphic units of the North Sea (formations and members) have been described within the context of this first-order tectono-stratigraphic framework. The correlation power of certain stratigraphic markers are also compared and contrasted, together with the potential cross-border equivalence of sedimentary units on different sides of the political median lines.

From seismic geomorphology to hydrostratigraphic units: spatial and temporal variations of deltaic to fluvial architecture, Pannonian Basin, Hungary

<p>This study focuses on the stratigraphic architecture of deltaic and fluvial sand lithologies within the Late Neogene Pannonian basin-fill succession in Hungary, identified from seismic and well data, in order to develop a quantitative hydrostratigraphic classification of the sequence. Hydrostratigraphic divisions are based on the hydraulic conductivity of the rock bodies, which depends on their extent, i.e. the thickness and the spatial distribution, as well as the lateral and vertical connectivity of sand bodies embedded in various muddy lithologies. Thus, we are going to build a simplified 3D lithological model for the uppermost 1500 m of the basin fill succession, that can later be transformed into hydrostratigraphic units and hydraulic conductivity values applied in a numerical flow model. The depositional environments change from deltaic to fluvial and within the fluvial system, the environment alternates between meandering and anastomosing. These intervals will appear as different hydrostratigraphic units in the model.  </p><p>In our work-flow, a merged three-dimensional seismic cube covering an area of approximately    50 x 40 km<sup>2</sup> was analyzed: 7 master horizons and several proportional slices were delineated in different attribute maps (e.g. amplitude, Root Mean Square amplitude, symmetry, similarity). These maps were generated to investigate the seismic geomorphological features and their associated depositional environments. Rock bodies were defined on the planform geometry of seismic attributes. Basic wireline logs (gamma, spontaneous potential, and resistivity) from 237 wells were interpreted simply in terms of sand, mud, and heterolithic muddy-sand, and finally were tied to the seismic cube. Lithology of rock bodies was determined with the help of well data. With this method, sandy deltaic lobes, sandy fluvial channel belts, and the muddy flood plains were identified. Based on the extension and density of sand bodies, percentages of sand vs clay (net-to-gross; N/G) as well as sand connectivity percentages were determined.</p><p>Above the deltaic succession, the fluvial depositional setting can be divided into three minor units. These units start with a meandering system, with 500-3600 m wide channel belts and a relatively high N/G. For an interval in the Pliocene about 350 m thick, a transition into an anastomosing river system is observed. This unit is characterized by channels about 100-200 m wide, with significantly lower N/G ratios and less connectedness. In the uppermost part of the succession, large meandering channel belts returned to the area. These changes in river style and paleo-hydrography affect the sand and clay ratio and their connectivity; therefore, definition of previous hydrostratigraphic units must be reconsidered. </p><p>This research is part of a project that has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 810980.</p>