ALBERT

Description: Fluviodeltaic stratigraphic architecture and its impact on fluid flow have been characterized using a high-resolution, three-dimensional, reservoir-scale model of an outcrop analog from the Upper Cretaceous Ferron Sandstone Member of central Utah. The model contains two parasequence sets (delta complexes), each with five or six parasequences, separated by an interval of coastal plain strata. Each parasequence contains one or two laterally offset teardrop-shaped delta lobes that are 6 to 12 km (4-7 mi) long, 3 to 9 km (2-6 mi) wide, 5 to 29 m (16-95 ft) thick, and have aspect ratios (width/length) of 0.4 to 0.8. Delta lobes have a wide range of azimuthal orientations (120{degrees}) around an overall east-northeastward progradation direction. In plan view, delta lobes in successive parasequences exhibit large (as much as 91{degrees}) clockwise and counterclockwise rotations in progradation direction, which are attributed to autogenic lobe switching. In cross-sectional view, parasequence stacking is strongly progradational, but a small component of aggradation or downstepping between parasequences reflects relative sea level fluctuations. We use flow simulations to characterize the impact of this heterogeneity on production in terms of the sweep efficiency, which is controlled by (1) the continuity, orientation, and permeability of channel-fill sand bodies; (2) the vertical permeability of distal delta-front heteroliths; (3) the direction of sweep relative to the orientation of channel-fill and delta-lobe sand bodies; and (4) well spacing. Distributary channel-fill sand bodies terminate at the apex of genetically related delta lobes and provide limited sand body connectivity. In contrast, fluvial channel-fill sand bodies cut into, and connect, multiple delta-lobe sand bodies. Low, but non-zero, vertical permeability within distal delta-front heteroliths also provides connectivity between successive delta-lobe sand bodies.

Description: Preferential dissolution of the biogenic carbonate polymorph aragonite promotes preservational bias in shelly marine faunas. While field studies have documented the impact of preferential aragonite dissolution on fossil molluscan diversity, its impact on regional and global biodiversity metrics is debated. Epicontinental seas are especially prone to conditions that both promote and inhibit preferential dissolution, which may result in spatially extensive zones with variable preservation. Here we present a multifaceted evaluation of aragonite dissolution within the Late Cretaceous Western Interior Seaway of North America. Occurrence data of mollusks from two time intervals (Cenomanian/Turonian boundary, early Campanian) are plotted on new high-resolution paleogeographies to assess aragonite preservation within the seaway. Fossil occurrences, diversity estimates, and sampling probabilities for calcitic and aragonitic fauna were compared in zones defined by depth and distance from the seaway margins. Apparent range sizes, which could be influenced by differential preservation potential of aragonite between separate localities, were also compared. Our results are consistent with exacerbated aragonite dissolution within specific depth zones for both time slices, with aragonitic bivalves additionally showing a statistically significant decrease in range size compared with calcitic fauna within carbonate-dominated Cenomanian–Turonian strata. However, we are unable to conclusively show that aragonite dissolution impacted diversity estimates. Therefore, while aragonite dissolution is likely to have affected the preservation of fauna in specific localities, time averaging and instantaneous preservation events preserve regional biodiversity. Our results suggest that the spatial expression of taphonomic biases should be an important consideration for paleontologists working on paleobiogeographic problems.