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Abstract

The Turonian-Coniacian Upper Ferron Sandstone Last Chance Delta was deposited along the western margin of the Western Interior Seaway as a wave-modified, river-dominated deltaic system. The Last Chance Delta was deposited during a slow relative sea-level rise whose rate of rise decreased with time. The sedimentation rate progressively decreased throughout the deposition of the Last Chance Delta.

Architectural and sedimentological data for deltaic near-marine sandstones indicate that primary deltaic depositional style is directly correlated with degree of wave-modification, which is controlled by the ratio of sedimentation rate to the rate of relative change in sea level. The progradational parase-quence sets have a mean sandstone dip length/thickness aspect ratio of 788. The aggradational parasequence sets are shorter with a mean length/thickness of 520. The retrogradational parase-quence sets are shorter and thinner with a mean length/thickness of 397. River-dominated progradational parasequences have a mean length/thickness of 611, a mean width/thickness of 212, and a mean length/width of 1.9. River-dominated, wave-modified progradational parasequences have longer dip lengths and a higher length/thickness of 845. The aggradational parasequences have similar lengths as the wave-modified parasequences, with a mean length/thickness of 606. The retrogra-dational parasequences are short and thin, with a mean length/thickness of 793.

Stream-mouth bar, reworked stream-mouth bar, and upper shoreface deposits show trends of length/thickness changing systematically with degree of wave-reworking, from a mean length/thickness of 479 (width/thickness = 256; length/width = 1.9) in river-dominated parasequences to 546 and 595 in reworked stream-mouth bar and upper shoreface deposits, respectively. Retrogradational parasequences have higher upper shoreface mean length/thickness aspect ratios of 649. Proximal delta-front, reworked proximal delta-front, and middle shoreface deposits show similar trends. River-dominated parasequences have mean proximal delta-front length/thickness of 425 (width/thickness = 472; length/width = 1.8) and reworked proximal delta-front and middle shoreface deposits have a mean length/thickness of 827 and 912, respectively. Retrogradational parasequences have a mean middle shoreface length/thickness of 807. Distal delta-front, reworked distal delta-front, and lower shoreface deposits also show similar trends. River-dominated parasequences have mean distal deltafront length/thickness ratios of 518 and reworked distal delta-front and lower shoreface deposits have mean length/thickness ratios of 819 and 2469, respectively. Retrogradational parasequences have a mean lower shoreface length/thickness of 981.

Architectural and sedimentological data for fluvial channel-belt sandstones indicate that over-all geometry, internal architecture, and preserved sedimentary structures are directly correlated with sedimentation rate and rate of relative change in sea level. Internal channel belt architecture is controlled by the response of the river equilibrium profile to changes in relative sea level and shoreline position. Channel belts, from progradational parasequence sets, deposited during times of high sedimentation rate and moderate relative sea-level rise, are laterally restricted and multi-storied with channel-fill elements stacked vertically within the channel-belt boundaries. Fluvial channel belts in the upper delta plain have average width/thickness aspect ratios of 28.8; distributary channel belts located near the paleoshoreline have average aspect ratios of 19.0. Fluvial channel belts from aggradational parasequence sets deposited during times when sedimentation rate was approximately equal to the rate of relative sea-level rise are laterally extensive and multi-storied with channel-fill elements stacked laterally en-echelon. Fluvial channel belts in the upper delta plain have average width/thickness aspect ratios of 59.2; distributary channel belts, located near the paleoshoreline have a mean aspect ratio of 12.1. Channel belts from retrogradational parasequence sets deposited during times when sedimentation rate was less than the rate of relative sea-level rise are laterally extensive and sheet-like with average aspect ratios of 100.0. Their channel-fill elements generally stacked vertically within the channelbelt boundaries. Amalgamated, braided fluvial deposits occur within small high-gradient incised valleys developed during periods of 4th- and 5th-order relative falls in sea level. The preserved incisedvalley fluvial deposits, within the Last Chance Delta, range in width from 1.3–8.8 km (0.8–5.5 mi) and in thickness from 9–32 m (27–96 ft); the average width/thickness aspect ratio is 169.4 near the valley mouths and 644.1 at 10–17 km (6–11 mi) inland from the mouth.

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