Gravity Tectonics on a Passive Margin: Deep Sea Drilling Project Site 415 in Relation to Regional Seismic Data

Abstract

Lithologic repetition and deformation of the cored sediments indicate that DSDP Hole 415A penetrated a 450-meter Upper Cretaceous allochthonous unit with several imbricate repetitions of a Cenomanian shale-and-carbonate sequence above a décollement of Albian and Cenomanian shale. This unit is expressed on multichannel seismic profiles as an interval of chaotic interval signal. Its upper surface is deformed by folds which predate the overlying Tertiary turbidites. Some of these folds maintain a harmonic profile in the sequence beneath the allochthonous unit, whereas others are restricted to the top of the unit. Although some folds are asymmetric, we cannot recognize a consistent vergence. Profiles beneath the continental slope east of Site 415 indicate that the drilled sequence is only the distal portion of a much more widespread feature. The equivalent sequence beneath the lower slope is thcker and is cut by several major, landward-dipping thrust faults across which there is a minimum of 20 km of overthrusting. The thrusts have a north-south strike, as do the disharmonic folds within each individual thrust sheet. Both folds and faults are refolded by a series of east-west-trending folds parallel to the Atlas tectonic trend. The displaced unit is not present beneath the upper slope, where it passes laterally into a prominent angular unconformity. This zone represents the source area for the overthrust masses. Onshore geological evidence indicates the emergence of the western High Atlas Mountains during Late Cretaceous times, and the allochthon is interpreted as an associated belt of gravity-driven overthrusts refolded by the last phases of this tectonism. Its scale is comparable with that of some overthrust zones in orogenic chains, and its existence on a passive margin may be important to the interpretation of exposed nappe complexes. Uplift of the western Atlas, gravity sliding, folding of the allochthon, and the onset of Canary Islands volcanism can be explained by sinistral strike-slip faulting along the South Atlas fault during Late Cretaceous time.

The Evolution of the Mazagan Continental Margin: A Synthesis of Geophysical and Geological Data with Results of Drilling during Deep Sea Drilling Project Leg 79

“…The prevalence of slump structures and other evidences of gravity sliding down the Mazagan Slope during the middle Cretaceous, and the occurrence of similar structures over a broad region to the south of the Mazagan sector, in the Essaouira sector, documented during DSDP Leg 50 (Price, 1980;Lancelot and Winterer, 1980), suggests that some regional tectonic steepening may have taken place during this general time period. Alternatively, the major slope steepening may have been during the time of most rapid subsidence, near the end of the Jurassic, when rotational block faulting resulted in differential rates of subsidence across the Mazagan Slope.…”

Breakup of Pangaea and plate kinematics of the central Atlantic and Atlas regions

“…It is interesting to note that these authors proposed a Kimmeridgian-Oxfordian reconstruction, which was based on the fit of the seaward edges of the JQZ south of the SAF and its North American counterpart (the so-called 40 • N fault). An interpretation of the Canary Islands as uplifted crustal blocks in a compressional or transpressive tectonic context was also stressed by Robertson & Stillman (1979), Price (1980), Fernandez et al (1997), Anguita & Hernan (2000) and Gutiérrez et al (2006). In summary, the available geological evidence supports the hypothesis that the NE-SW oriented Canary Islands Ridge is a compressive structure linked to the Atlas orogeny.…”

Late Cretaceous to Cenozoic geodynamic evolution of the Atlantic margin offshore Essaouira (Morocco)

“…The Jurassic post-rift passive margin environment was characterised by the development of carbonate platforms and ramps, which formed on top of the evaporites in continental Morocco, and by deposition of marine mudstones in the more distal margin areas. Valanginian to Hauterivian turbidite sequences have been encountered by well DSDP 416 (Price, 1980) and are also exposed along the Moroccan coast (Saadi et al, 1978) and on Fuerteventura Island (Steiner et al, 1998). Sediments along the slopes and in the deep basin areas became generally more fine-grained, with turbidite sequences forming during the Tithonian to Hauterivian, and the accumulation of predominantly claystones from the Barremian to the Albian (DSDP wells 370, 415 and 416; Lancelot & Winterer, 1980a Early Cretaceous (Bertotti et al, 2010), associated with erosion of clastic material and transport to the Moroccan deep-water offshore.…”

“…The Miocene to recent sediments (average sedimentation rate of 18-20 m Ma À1 ; The Shipboard Scientific Party, 1980a) comprise chalk and marls. However, the thick Cenomanian succession in well DSDP 415 has been studied in detail in the context of gravity sliding by Price (1979b) and Dunlap et al (2010). Below the Middle Eocene unconformity, a few tens of metres of Eocene and about 150 m of Palaeocene mudstone (average sedimentation rate of 15 m Ma À1 ; The Shipboard Scientific Party, 1980a) are present.…”