Sequence stratigraphy of the Mancos Shale,lower Tres Hermanos Formation,and coeval middle Cenomanian to middle Turonian strata,southern New Mexico,USA

Sequence stratigraphic analysis of four widely spaced outcrops of middle Cenomanian to middle Turonian strata deposited in the Western Interior foreland basin in southern New Mexico, USA, defines ten sequence boundaries in a marine shale‐rich interval ca 200 m thick. The majority of sequence boundaries are based on basinward shifts in lithofacies characterized by either a non‐Waltherian contact between distal‐bar or lower shoreface sandstone and underlying lower offshore shale, or an erosional contact between distal‐bar or lower shoreface sandstone and underlying upper offshore shale. The sequence boundaries commonly correlate basinward to packages of storm‐deposited sandstone and to beds of sandy grainstone composed of winnowed inoceramid shell fragments. In several cases, however, the sequence boundaries pass basinward into presumably conformable successions of lower offshore shale. Maximum flooding surfaces within the sequences are represented by one or more beds of locally phosphatized globiginerid wackestone and packstone or exist within a conformable succession of lower offshore shale. Following initial south/south‐westward transgression into the study area, the regional trend of palaeoeshorelines was north‐west to south‐east, although isopach data indicate that lobes of sandstone periodically spread south‐eastward across the study area. The ten sequences in the study area are arranged into a third‐order composite megasequence that is characterized by overall upward‐deepening followed by upward‐shallowing of sequences. The composite megasequence is similar but not identical to the previously established T‐1 transgression and R‐1 regression in New Mexico. Based on radioisotopic dates of bentonites, the average frequency of the sequences within the study area was ca 327 kyr, which is consistent with fourth‐order cycles of ca 400 kyr interpreted in coeval marine strata elsewhere in the world.     

Sediment dispersal and organic carbon preservation in a dynamic mudstone‐dominated system,Juana Lopez Member,Mancos Shale

A balance between primary production, rates of sediment accumulation or dilution, and biological or diagenetic destruction has long been considered a key control on organic carbon preservation in modern offshore marine environments. Additionally, current understanding of sediment transport processes in offshore environments has advanced in the last decade to include variable energy and dynamic mechanisms, requiring a re‐evaluation of ancient deposits in these systems. The Juana Lopez Member of the Mancos Shale preserves organic carbon‐rich mudstone interbedded and interlaminated with sandstone that records high energy traction flow conditions. Core, outcrop and geochemical data from the Juana Lopez Member were used to elucidate sediment provenance and processes controlling organic carbon preservation and distribution in this mudstone‐dominated system. Five dominant lithofacies with varying grain size, sedimentary fabrics, composition and grain origins were differentiated and were deposited in three main environments: the prodelta, fringe zone and low angle offshore ramp. Basal deposits of the Juana Lopez Member consist of siliceous sandstone‐dominated, heterolithic deposits with characteristic sedimentary structures (for example, current ripples and normal grading) that indicate offshore‐directed underflows, or hyperpycnites, delivered from the updip Ferron/Frontier deltaic system. In the upper portion of the Juana Lopez Member, a compositional change to biogenic carbonate‐rich sandstone and mudstone is interpreted to be as a result of increased accommodation in central Utah (USA), associated base‐level rise and shoreline‐parallel sediment transport. Non‐parallel laminated, organic carbon‐rich mudstone is preserved throughout the Juana Lopez Member. Depositional fabrics and trace element signatures suggest that these deposits are the result of dynamic conditions at the sea floor and in the oxic to suboxic water column, further challenging the notion that organic‐bearing mudstone is deposited solely through suspension settling in anoxic waters. Punctuated delivery of organic carbon laden sediment from mixed terrestrial and marine sources resulted in an event‐bed style of organic carbon deposition and preservation.     

A depositional model for offshore deposits of the lower Blue Gate Member,Mancos Shale,Uinta Basin,Utah, USA

Depositional models that use heterogeneity in mud‐dominated successions to distinguish and diagnose environments within the offshore realm are still in their infancy, despite significant recent advances in understanding the complex and dynamic processes of mud deposition. Six cored intervals of the main body of the Mancos Shale, the lower Blue Gate Member, Uinta Basin, were examined sedimentologically, stratigraphically and geochemically in order to evaluate facies heterogeneity and depositional mechanisms. Unique sedimentological and geochemical features are used to identify three offshore environments of deposition: the prodelta, the mudbelt and the sediment‐starved shelf. Prodelta deposits consist of interlaminated siltstone and sandstone and exhibit variable and stressed trace fossil assemblages, and indicators of high sedimentation rates. The prodelta was dominated by river‐fed hyperpycnal flow. Mudbelt deposits consist of interlaminated siltstone and sandstone and are characterized by higher bioturbation indices and more diverse trace fossil assemblages. Ripples, scours, truncations and normally graded laminations are abundant in prodelta and mudbelt deposits indicating dynamic current conditions. Mudbelt sediment dispersal was achieved by both combined flow above storm wave base and current‐enhanced and wave‐enhanced sediment gravity flows below storm wave base. Sediment‐starved shelf deposits are dominantly siltstone to claystone with the highest calcite and organic content. Bioturbation is limited to absent. Sediment‐starved shelf deposits were the result of a combination of shelfal currents and hypopycnal settling of sediment. Despite representing the smallest volume, sediment‐starved shelf deposits are the most prospective for shale hydrocarbon resource development, due to elevated organic and carbonate content. Sediment‐starved shelf deposits are found in either retrogradational to aggradational parasequence sets or early distal aggradational to progradational parasequence sets, bounding the maximum flooding surface. An improved framework classification of offshore mudstone depositional processes based on diagnostic sedimentary criteria advances our predictive ability in complex and dynamic mud‐dominated environments and informs resource prospectivity.