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Rocks of the mid-Cretaceous Colorado allogroup and time-equivalent strata (late middle Albian to early Campanian; $104 Ma to $83 Ma), are dominated by marine mudstone and siltstone that was deposited in a few tens of meters of water up to several hundred kilometers from shore. In the north and west, nearshore and coastal plain facies form relatively minor components of the allogroup. The rocks are divided into allomembers by marine transgressive and flooding surfaces. Allomembers are the fundamental genetic stratal packages, and typically span $50-200 kyr. Allomembers are grouped into larger informal "units" (spanning $400-800 kyr), and alloformations (spanning a few Myr). Except for the Cenomanian Dunvegan alloformation, rocks of the Colorado allogroup lack well-developed clinoform stratification. The scarcity of clinoforms suggests that supply rate usually exceeded accommodation rate and areas of subsidence were immediately filled with sediment up to a subaqueous "accommodation envelope" defined by effective wave base. Alloformations form prismatic wedges, hundreds of kilometers in dip and strike extent and 100-900 m thick. Their component "units" have more arcuate depocenters typically 100-300 km in strike length, suggestive of more localized loading. Lateral shifts of depocenters and rotation of isopachs between units suggest that the loci of subsidence, loading, and of inferred active thrust advance, shifted laterally over distances of $100-300 km in (1 my. Allomembers, occupying even more localized depocenters of 100-200 km strike length, may shift along strike by as much as 300 km on the timescale of a single marine flooding surface (i.e., < $10 kyr), suggesting that the emplacement of loads was spatially and temporally very non-uniform.Successive allomembers commonly change upward from wedge-to sheet-shaped rock bodies. Wedges indicate high accommodation and aggradation rates that confined nearshore sandstone and conglomerate to a belt close to the orogenic margin. In contrast, sheets commonly contain highly progradational nearshore sandstone and represent periods of low accommodation rate that favored shallow water and rapid regression.Two principal depocenters are recognized in the Colorado allogroup. A N-S trending depocenter in northern Alberta and British Columbia accommodated Albian to middle Cenomanian rocks; much of this time is represented by unconformities in the south. In the south, a NW-SE trending depocenter was initiated in the late Cenomanian at $95 Ma and accommodated late Cenomanian to early Campanian (and younger) rocks; in the northern depocenter, late Cenomanian to late Coniacian time is represented by an unconformity. The switch between the northern and southern depocenters apparently Tectonics of Sedimentary Basins: Recent Advances, First Edition. Edited 480 took place very abruptly at $95 Ma. We speculate that subsidence of the northern depocenter was related to the collision and clockwise rotation of the Stikine and Yukon-Tanana terranes during their impingement upon the adjoining ...

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A black marine mudstone in allomember A of the middle Cenomanian Dunvegan Formation, Alberta, yielded a well-preserved, deep-bodied fish skeleton attributable to a new genus and species, Tycheroichthys dunveganensis, in the Paraclupeidae (Clupeomorpha, Ellimmichthyiformes). This new taxon appears most closely related to members of the subfamily Paraclupeinae, which includes largely freshwater and estuarine or marginal marine species known from South and Central America and China. The Dunvegan Formation represents a large delta complex deposited in the Western Interior Seaway at about 65°N, where mean sea water temperature may have been about 10 °C. The specimen was found about 60 km from the contemporaneous shoreline, and, in life, apparently inhabited a shallow, muddy pro-delta environment characterized by high turbidity and variable salinity. The fish may have died when it was engulfed by brackish floodwater that also introduced suspended mud that rapidly buried the body, preventing physical disarticulation of the skeleton. River outflow probably produced a salinity- and density-stratified water column that limited oxygen transfer to the sea floor, leading to dysaerobic bottom water and an absence of scavengers. High terrestrial phytodetrital content of the sediment also favoured anaerobic conditions that limited bacterial decay.

After prograding for several hundred kilometres during Middle Cenomanian time, the Dunvegan delta complex in north‐west Alberta and adjacent British Columbia experienced stepwise transgression, commencing at about the Middle to Late Cenomanian boundary. Progressive drowning of the delta complex is recorded by Dunvegan allomembers B and A, each comprised of three simple depositional sequences, bounded by composite subaerial unconformity/flooding surfaces. Each sequence represents an array of deltaic depositional environments. Delta‐front sandstones preserve little evidence, such as hummocky cross‐stratification, for powerful storm wave action, although wave and combined‐flow ripples are common. Delta‐front sandstone bodies tend to be smaller and lobate in the lower part of the studied interval, and larger and more linear near the top. This suggests increasingly effective wave‐driven redistribution of sand as more open‐marine conditions were gradually established. The top surfaces of allomembers B and A are locally incised by sandstone‐filled palaeovalleys up to 19 m deep; river incision may have been a response to relative sea‐level fall and/or a change in the ratio of discharge to sediment load. Overall, the shoreline described a broad arc, open to the south east, with the sense of shoreline migration north‐west to south‐east. For each sequence, the shoreline migrated an average of 80 km between transgressive and regressive limits. The transgressive limit shows a progressive landward offset of about 15 km per sequence, culminating in complete drowning of the delta system above sequence A3. Isopach maps show that syn‐depositional tectonic subsidence rotated the basin down to the south‐west; palaeogeographic maps show, however, that the sea floor sloped to the south‐east, implying that sediment redistribution effectively filled all tectonically generated accommodation and maintained a south‐east inclined depositional surface. Transgressions and regressions across this surface were therefore driven primarily by eustasy rather than pulses of tectonic subsidence. Simple calculations based on inferred alluvial gradients of 10–20 cm/km suggest that eustatic excursions of ca 8–16 m would have been sufficient to generate sequence thicknesses on the order of 10 m. Limited geochronologic and biostratigraphic control suggests that the six simple sequences that form Dunvegan allomembers B and A each represent an average of about 41 kyr, suggesting that the orbital obliquity cycle was the primary control on high‐frequency sea‐level cycles.