4 research outputs found
From river to delta: down-dip changes in facies, architecture, and key stratigraphic surfaces in a low-accommodation setting
This PhD thesis is a collection of four articles that investigate the influence of a low-accommodation setting on down-dip changes in facies, architecture and key stratigraphic surfaces in fluvio-deltaic transects. The study utilizes the Cenomanian Mesa Rica Sandstone (Dakota Group), which represents contemporaneous fluvial and deltaic deposition in the Western Interior Basin. The ~400 km transect is exhumed along depositional profile, from southeast Colorado to central-east New Mexico. These excellent conditions allowed both regional-scale and high-resolution studies of key-areas. The results reveal a sand-rich end-member example of fluvial and deltaic deposition in a low-accommodation setting. The thesis emphasizes that such low-accommodation settings favour accelerated avulsion frequencies, lowered preservation potential, and formation of laterally extensive stratigraphic surfaces and sheet-like sandstone bodies. Several key stratigraphic surfaces were generated throughout a sea-level cycle, which highlights that erosion and deposition occurred contemporaneously both at regional and local scale
Internal mouthâbar variability and preservation of subordinate coastal processes in lowâaccommodation proximal deltaic settings (Cretaceous Dakota Group, New Mexico, USA)
Mouth bars are the fundamental architectural elements of proximal deltaic successions. Understanding their internal architecture and complex interaction with coastal processes (fluvial, tide and waveâdominated) is paramount to the interpretation of ancient deltaic successions. This is particularly challenging in lowâaccommodation systems, because they are commonly characterized by thin, condensed and topâtruncated sections. This study analyses the exhumed Cenomanian Mesa Rica Sandstone (Dakota Group, Western Interior Seaway, USA), a fluvioâdeltaic system covering a ca 450 km depositional dipâparallel profile. The study targets the proximal deltaic expression of the system, using 22 sedimentary logs (total of 390 m) spatially correlated within a ca 25 km2 study area at the rim of the Tucumcari Basin. Analysis of facies distributions, depositional architecture and spatial extent of stratigraphic surfaces reveals a 6â10 m thick, sharpâbased and sandâprone deltaic package, comprising several laterally extensive (>1.4 km width) mouth bars. Composite erosional surfaces infilled with multiâstorey fluvial and marineâinfluenced channel deposits (12â20 m thick, 100â250 m wide) scour locally into the deltaic package. Based on differences in sedimentary structures, bed thicknesses, occurrence of interflood beds and bioturbation indexes, four different subâenvironments within single mouth bars were distinguished. These range from mouthâbar axis, offâaxis, fringe to distalâfringe deposits, which reflect waning depositional energy with increasing distance from the distributary channel mouth. The interpreted mouthâbar components also show internal variability in dominant process regime, with overall river dominance but local preservation of tide influence in the fringe and distal fringe components. Mouthâbar deposits amalgamate to form an extensive sandârich sheet body throughout the study area, in which interflood mudstone to veryâfine grained sandstone beds are nearly absent. These features reflect successive coalescence of mouth bars in a low accommodation/supply (A/S) setting. These conditions promoted recurrent channel avulsion/bifurcation and thus the potential reworking of previously deposited mouthâbar fringe and distalâfringe sediments, where time and background processes are better recorded. Results of this study evidence internal processâregime variability within mouthâbar components. They also caution against the possible loss of preservation of subordinate coastal processes (e.g. tidal indicators), and consequent underestimation of the true mixed influence in lowâaccommodation deltaic settings
Lowâaccommodation and backwater effects on sequence stratigraphic surfaces and depositional architecture of fluvioâdeltaic settings (Cretaceous Mesa Rica Sandstone, Dakota Group, USA)
The adequate documentation and interpretation of regionalâscale stratigraphic surfaces is paramount to establish correlations between continental and shallow marine strata. However, this is often challenged by the amalgamated nature of lowâaccommodation settings and control of backwater hydraulics on fluvioâdeltaic stratigraphy. Exhumed examples of fullâtransect depositional profiles across riverâtoâdelta systems are key to improve our understanding about interacting controlling factors and resultant stratigraphy. This study utilizes the ~400 km transect of the Cenomanian Mesa Rica Sandstone (Dakota Group, USA), which allows mapping of downâdip changes in facies, thickness distribution, fluvial architecture and spatial extent of stratigraphic surfaces. The two sandstone units of the Mesa Rica Sandstone represent contemporaneous fluvioâdeltaic deposition in the Tucumcari subâbasin (Western Interior Basin) during two regressive phases. Multivalley deposits pass downâdip into singleâstory channel sandstones and eventually into contemporaneous distributary channels and deltaâfront strata. Downâdip changes reflect accommodation decrease towards the paleoshoreline at the Tucumcari basin rim, and subsequent expansion into the basin. Additionally, multiâstorey channel deposits bound by erosional composite scours incise into underlying deltaic deposits. These represent incisedâvalley fill deposits, based on their regional occurrence, estimated channel tops below the surrounding topographic surface and coeval downstepping deltaâfront geometries. This opposes criteria offered to differentiate incised valleys from floodâinduced backwater scours. As the incised valleys evidence relative seaâlevel fall and floodâinduced backwater scours do not, the interpretation of incised valleys impacts sequence stratigraphic interpretations. The erosional composite surface below fluvial strata in the continental realm represents a sequence boundary/regional composite scour (RCS). The RCSâ diachronous nature demonstrates that its downâdip equivalent disperses into several surfaces in the marine part of the depositional system, which challenges the idea of a single, correlatable surface. Formation of a regional composite scour in the fluvial realm throughout a relative seaâlevel cycle highlights that erosion and deposition occur virtually contemporaneously at any point along the depositional profile. This contradicts stratigraphic models that interpret lowâaccommodation settings to dominantly promote bypass, especially during forced regressions. Sourceâtoâsink analyses should account for this in order to adequately resolve timing and volume of sediment storage in the system throughout a complete relative seaâlevel cycle
Supradetachment to rift basin transition recorded in continental to marine deposition; Paleogene Bandar Jissah Basin, NE Oman
A transition from supradetachment to rift basin signature is recorded in the ~1,500 m thick succession of continental to shallow marine conglomerates, mixed carbonateâsiliciclastic shallow marine sediments and carbonate ramp deposits preserved in the Bandar Jissah Basin, located southeast of Muscat in the Sultanate of Oman. During deposition, isostaticallyâdriven uplift rotated the underlying Banurama Detachment and basin fill ~45° before both were cut by the steep Wadi Kabir Fault as the basin progressed to a riftâstyle bathymetry that controlled sedimentary facies belts and growth packages. The upper Paleocene to lower Eocene Jafnayn Formation was deposited in a supradetachment basin controlled by the Banurama Detachment. Alluvial fan conglomerates sourced from the Semail Ophiolite and the Saih Hatat window overlie the ophiolitic substrate and display sedimentary transport directions parallel to tectonic transport in the Banurama Detachment. The continental strata grade into braidplain, mouth bar, shoreface and carbonate ramp deposits. Subsequent detachmentârelated folding of the basin during deposition of the Eocene Rusayl and lower Seeb formations marks the early transition towards a riftâstyle basin setting. The folding, which caused drainage diversion and is affiliated with sedimentary growth packages, coincided with upliftâisostasy as the Banurama Detachment was abandoned and the steeper Marina, Yiti Beach and Wadi Kabir faults were activated. The upper Seeb Formation records the late transition to riftâstyle basin phase, with faultâcontrolled sedimentary growth packages and facies distributions. A predominance of carbonates over siliciclastic sediments resulted from increasing nearâfault accommodation, complemented by reduced sedimentary input from upland catchments. Hence, facies distributions in the Bandar Jissah Basin reflect the progression from detachment to riftâstyle tectonics, adding to the understanding of postâorogenic extensional basin systems