Abstract The Late Turonian–Early Coniacian upper Ferron Sandstone Last Chance Delta was built on the western margin of the Western Interior Seaway, as a wave-modified, river-dominated deltaic system during a slow third-order relative sea-level rise which was interrupted by minor fourth-order and fifth-order relative falls in sea level. Sediment supply declined steadily during the deposition of the Last Chance Delta. Four fourth-order depositional sequences are recognized (denoted FS1–FS4). These four depositional sequences are separated by fluvially eroded unconformities with up to 25–32 m of erosional relief. These unconformities record forced regression and locally the development of incised-valley systems. The oldest incised-valley system occurring at the base of sequence FS2, within the lower, highly progradational part of the Last Chance Delta, was filled during a complete fifth-order sea-level cycle when the sedimentation rate exceeded the rate of relative rise in sea level. The sediment and accommodation-space partitioning during the filling of the incised-valley was such that the valley was filled during a complete sea-level cycle. The incised-valley system occurring at the base of sequence FS3 was filled on the rising limb of a fourth-order sea-level cycle and sedimentation rate was balanced with the rate of relative rise in sea level. The valley was completely filled before the sea-level cycle was two-thirds complete. The result was that the valley contained only lowstand and minor transgressive systems tract deposits and that the highstand sediments were deposited above the incised valley, during the final third of the sea-level cycle. The incised-valley system occurring at the base of sequence FS4 was filled along the lower rising limb of a fourth-order sea-level cycle and when sedimentation rate was slightly less than the rate of relative rise in sea level. Although slow, the sedimentation rate allowed the valley to be completely filled before the sea-level cycle was 10% complete. The result was that the valley contained only lowstand and the very earliest transgressive systems tract estuarine deposits and that the transgressive, retrogradational paralic parasequences were deposited above the incised valley, during the transgressive part of the sea-level cycle. Variations in sedimentation rate and the rate of relative rise in sea level result in different depositional facies tracts within incised valleys and control the rate of the valley filling and the partitioning of accommodation space. These parameters, coupled with the maximum incision depth, govern the duration of the valley-filling episode. The duration of valley-filling event relative to the total duration of the sea-level cycle controls the presence or absence of systems tracts within the incised valley. The incised-valley-fill deposits of the upper Ferron Sandstone Last Chance Delta serve as models for understanding and predicting the internal architecture and sedimentology of incised valleys, filled during a range of conditions of sedimentation rate and relative sea-level rise and during sea-level cycles of different durations.

Abstract The deposition of the late Turonian-early Coniacian Upper Ferron Sandstone Last Chance Delta occurred during a long slow relative rise in sea level, interrupted only by three minor 4th-order relative falls in sea level. An analysis of stratigraphic, geometric, and architectural data, for both near-marine and non-marine facies indicate that Last Chance Delta architecture was controlled by changes in sediment supply. The systems tract style and parasequence set stacking pattern within deposition-al sequences reflect the relationship of the rate of sedimentation to the rate of relative change in sea level. The Upper Ferron Sandstone Last Chance Delta was deposited along the western margin of the Cretaceous Western Interior Seaway between 90.3-88.6 Ma, as a wave-modified, river-dominated fluvial-deltaic system. Detailed stratigraphy and quantitative cross sections based on volcanic ash layer correlations and coal-zone stratigraphy have been used to delineate the depositional sequence stratigraphy of the Last Chance Delta clastic wedge exposed in Castle Valley of east-central Utah. Relative sea level and local subsidence curves, and sedimentation rates have been determined from analysis of these cross sections. Within the Last Chance Delta, at least 42 parasequences organized into 14 parasequence sets form four 4th-order depositional sequences (denoted FS1 through FS4). In the non-marine to transitional near-marine facies associations, the upper boundaries of parasequence sets, when not coincident with sequence boundaries or transgressive ravinement surfaces, are coal zones. Laterally extensive unconformities, with 20–30 m (66-98 ft) of erosional relief, locally mark the lower boundaries of sequences FS2, FS3, and FS4. These unconformities are interpreted as type-1 sequence boundaries that record basinward shifts of paleoshorelines by up to 3-7 km (2–4 mi) and the development of incised-valley systems ranging up to 6-10 km (4-6 mi) wide. The lower boundary of sequence FS1 is a correlative conformity immediately below a condensed section within the underlying Tununk Shale. The upper boundary of FS4 is a correlative conformity stratigraphically above a concretion-bearing condensed section a few meters above the uppermost sandstones of sequence FS4. FS1 is a 4th-order highstand sequence of the 3rd-order Hyatti Composite Sequence. FS2 and FS3 are the progradational (early lowstand) to aggradational (late lowstand) sequences of the Ferron Composite Sequence; FS4 is the transgressive sequence. Lowstand sequence FS2 contains well-defined forced regression deposits and higher-order sequence boundaries. The highstand deposits of the Ferron Composite Sequence are represented, within Castle Valley, only by offshore marine shale deposits.

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.