The Lower Colorado Group (Late Albian-earliest Cenomanian) has been allostratigraphically divided on the basis of regional unconformities and transgressive surfaces, resulting in recognition of an informal Lower Colorado allogroup, comprising the Paddy, Joli Fou, Viking, Westgate and Fish Scales alloformations. The Paddy alloformation forms an eastward-thinning wedge up to 125 m thick, composed of nine allomembers that progressively onlap the basal unconformity PE0 from west to east. Paddy rocks are mainly alluvial in the west, grading into marginal marine in the east and north. Paleo valley-fills are present at the tops of most allomembers. The Joli Fou alloformation transgressively overlies nonmarine Mannville Group rocks, and forms a relatively sheet-like blanket (average 20 m) of marine mudstone that coarsens in the north, where it is assigned to the lithostratigraphic Viking Formation, and in the far south, where it is assigned to part of the lithostratigraphic Bow Island Formation. The Viking alloformation erosively overlies the Joli Fou alloformation at surface VE0 and consists of regional allomembers VA, VB and VD, separated by unconformities VE0, VE1, VE3 and VE4. Allomembers VA and VB (mean 30 m thick) are intensely bioturbated shallow marine silty fine sandstones whereas allomember VD is weakly bioturbated, and includes sandy shoreface deposits in the SW and thick (>50 m) offshore marine mudstone in the north (part of the lithostratigraphic Hasler Formation). Allomember VC is confined to paleovalley deposits below VE3. All regional Viking allomembers can be traced into the lower Bow Island Formation in the south. Marine mudstones of the Westgate alloformation form a wedge thinning from >600 m in the NW to <40 m in the far south, comprising informal units WA, WB and WC from base to top. Westgate strata onlap southward onto VE4 such that only unit WC persists to southern Alberta, where it passes into marginal marine facies of the middle and upper Bow Island Formation. The Fish Scales alloformation (earliest Cenomanian) erosively overlies the Westgate alloformation at surface FE1 and comprises two units FA and FB, the former being confined to a depocentre in the NW. The Fish Scales alloformation is characterized by abrupt introduction of fine sand into the basin and sea-floor erosion which formed uranium-enriched phosphatic lags which give a characteristic highly radioactive log signature. An absence of benthic fauna and high organic content indicate deposition below anoxic water. The top of the Fish Scales alloformation is the Fish Scales Upper (FSU) marker which is a highly radioactive condensed section and downlap surface below prograding clinothems of the early-mid Cenomanian Dunvegan alloformation. Allomember FB is locally coarse-grained in the far south, forming the lower part of the Barons Sandstone, whereas the upper Barons is fine grained and equivalent to allomember C of the Dunvegan Formation.The Paddy alloformation is entirely, or almost entirely older than the Joli Fou alloformation, and henc...

No abstract

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 ...

Continental successions of the North American Western Interior retroarc foreland basin provide an excellent opportunity to evaluate the tectonic controls on nonmarine sequence stratigraphy. The transition between the Upper Jurassic Brushy Basin Member anastomosed fluvial system of the Morrison Formation and the gravelly braided-river deposits of the Buckhorn Conglomerate has been studied to assess the dispersal of coarse clastics and the development of associated basin-wide unconformities in a sequence stratigraphic framework. The sharp contact between the two members is interpreted to be conformable based on stratigraphic, sedimentologic, and petrologic data collected at and near Cedar Mountain in central Utah, while a regional, mature paleosol at the top of the Buckhorn Conglomerate indicates the presence of a major sequence boundary. These interpretations are combined with paleoflow data and fluvial architectural analysis to reconstruct the evolution of the alluvial equilibrium profiles that controlled deposition of the succession. The Buckhorn Conglomerate is interpreted as an example of a post-orogenic deposit on the basis of (1) its tabular geometry, (2) distance from the contemporaneous thrust belt, (3) shift to transverse paleoflow direction from the preceding Brushy Basin axial drainage, (4) conformable lower contact, and (5) presence of an overlying sequence boundary with a westward-expanding hiatus eroding the coeval foredeep. The onset of a protracted period of low rate of orthogonal convergence between the Farallon and North American Plates resulted in tectonic quiescence and consequent isostatic rebound of the Middle to Late Jurassic thrust belt and the contiguous contemporaneous foredeep, as well as dynamic uplift. The ensuing basin-wide decrease in subsidence rate induced the progradation of the Buckhorn alluvial plain into the Morrison back-bulge depozone, and the subsequent generation of the overlying pedogenic unconformity, which separates the deposits of the Jurassic flexural event from the overlying Sevier tectonostratigraphic unit.

No abstract