Sequence stratigraphic architecture of marine to fluvial deposits across a passive margin (Cenomanian, Atlantic margin, Morocco, Agadir transect)

Abstract

Seven sections, covering the upper Albian to lowermost Turonian, have been correlated from full-marine to continental-dominated deposits across a passive margin, along a transect 425 km long, from the present-day Atlantic coast to the "Pre-African Trough" between the Anti-Atlas and the HighAtlas. The thickness of the Cenomanian succession changes from around 500 metres in the fully marine sections to 250 metres in mostly continental facies in the western High-Atlas, about 150 km updip, to a few tens of metres in the Bou Tazoult area. The strata thicken again eastwards into the Pre-African Trough where they can be traced without major facies changes to the Kem Kem embayment and to the Bechar area in Algeria. Over all this eastern area, continental facies are overlain by the fully-marine shallow-water deposits of the Cenomanian-Turonian boundary interval. A first major conclusion is that fluvial aggradation in high-frequency transgressive-regressive sequences is coeval with the seaward-shift of the shoreline, in accordance with the genetic sequence stratigraphic model of GALLOWAY (1989). Both the flatness of the depositional profile and the corresponding very low energy of the marine environment during the transgressions account for the blanket of red continental clays on top of marine facies in updip depositional sequences, which is then preserved under the marine transgressive surface of the next sequence. Carnets Geol. 15 (12)A second major conclusion is that the high-frequency transgressive-regressive (T-R) sequences do not look like classical parasequences bounded by transgression surfaces. They usually exhibit a surface created by a sea-level fall within the regressive half-cycle. This is interpreted in the following way: regressions did not operate through a regular seaward-shift of the shoreline, but through stepped sealevel falls. The very low slope of the depositional ramp is thought to have enhanced the sequence stratigraphic record of such stepped regressions. Short-term, high-frequency sequences are organized into medium-frequency T-R sequences (seven in the Cenomanian) which show an overall aggrading and slowly retrograding pattern along the whole transect.Comparisons with other basins show that medium-frequency sequences do not fit the third-order depositional sequences described elsewhere, casting doubts about a eustatic mechanism for their deposition. Tazoult), sur environ 250 km. La série s'épaissit à nouveau vers l'est dans le sillon pré-africain où elle peut être suivie sans changements notables vers le golfe des Kem-Kem et le secteur de Béchar en Algérie. Sur toute la partie orientale de la transversale, les faciès continentaux ou mixtes sont recouverts par les dolomies marines du passage Cénomanien-Turonien. Une première conclusion majeure est que, dans les séquences transgression-régression (T-R) à haute fréquence, l'aggradation fluviatile accompagne sans hiatus le déplacement de la ligne de rivage au cours du demi-cycle régressif, en accord avec le modèle génétique de GALLOWAY (...

Geology and paleontology of the Upper Cretaceous Kem Kem Group of eastern Morocco

“…Farther to the west on the flanks of the High Atlas near Tinghir, a similarly more complete section is present and was visited by one of us (DBD). Two units are present under the carbonate platform that correspond well with the two units of the Kem Kem beds (Gauthier 1960, Essafraoui et al 2015. The finer-grained upper unit, nevertheless, has more marine marls, marine sandstones, a beach sandstone, and gypsiferous evaporate beds than typical in the corresponding Kem Kem unit (Essafraoui et al 2015: fig.…”

“…These deposits of Cenomanian age, although much thinner (~ 10 m) than the Kem Kem beds, have yielded a fauna broadly comparable to that reported here from the Kem Kem beds . These data led Essafraoui et al (2015) to conclude that a north-dipping, low-gradient ramp also existed during the Cenomanian on the Algerian side of the western Saharan craton.…”

“…Choubert (1952) and Lavocat (1954a), however, assigned the Kem Kem beds to the "infracénomanien," suggesting a probable Albian age. Although ammonites and other nonvertebrate fossils have established the Late Cenomanian-Turonian age of the overlying limestone complex (Ferrandini et al 1985;Ettachfini and Andreu 2004;Ettachfini et al 2005;Essafraoui et al 2015), no fossils were known at that time from the Kem Kem beds that could be reasonably associated with stage-level temporal resolution.…”

“…Transgressive conditions, nonetheless, characterize all of these sections, which grade upward from shallow-water coastal facies to a more massive, deeper-water (subtidal) carbonate platform (Ferrandini et al 1985). The platform complex also markedly thickens to the north of what Essafraoui et al (2015) identify as the "Kem Kem embayment", on which the fluvio-deltaic Kem Kem beds were deposited. The platform is typically ~ 30 m thick over the Kem Kem beds.…”

“…2). The Kem Kem embayment is bounded to the west by the Anti-Atlas and to the east by the Bechar promontory (Essafraoui et al 2015). The "Taouz Basin" (Cavin et al 2010) is another name applied to the structural depression that received sediments in the Kem Kem region.…”