English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

During Leg 79 Jurassic limestones were recovered at four sites drilled at the front of the Mazagan Plateau, making it possible to reconstruct depositional conditions at the outer edge of the continental margin. At Site 544, red Oxfordian limestones overlie red continental elastics and Paleozoic basement. The limestones are composed of cyanobacterial crusts grading upward into bioclastic grainstones and wackestones. Early lithification is evidenced by the development of hardgrounds and cavities as the depositional environment deepened and the sedimentation rates decreased. At Site 545 Jurassic limestones occur as constituents of debris flows, which are embedded with Miocene nannofossil oozes. The rock fragments were derived from oolitic shoals, coral bioherms, and deeper shelf deposits. Calpionellids indicate a Tithonian to Early Cretaceous age for the clasts. Some of these limestone breccias have a complex history. The first fragmentation occured during the Early Cretaceous, the second during the Late Cretaceous, and the final emplacement happened during the late Miocene. In situ limestones were encountered beneath the Cenozoic cover at Site 545 and indicate the presence of a retreating carbonate platform during the Late Jurassic with a downfaulted platform edge. The uppermost portion is composed of brecciated lagoonal carbonates forming a periplatform talus in front of the developing Mazagan Escarpment. The oldest sediments recovered at Site 545 are sandy slope deposits of Middle Jurassic age. In the middle of the section, dolomitic stromatolites indicate brief regressive intervals. The presence of a poorly lithified talus at the edge of the platform confirms the initial drowning phase of the carbonate platform, which occurred before the final cementation of the Jurassic platform deposits. At Site 546, located upon a salt dome, two fragments of Jurassic oolitic limestone were embedded into middle Miocene calcareous ooze. Their presence marks the tail of a debris flow. Site 547, which is located in a small structural sub-basin at the base of the margin, demonstrates that marine deposition started with the development of stromatolites overlying (?)Triassic continental red beds. A rapid change to pelagic conditions is documented by the deposition of micritic nodular limestones. Black shale interlayers suggest that restricted conditions occasionally developed in the sub-basin, which was filled by sandy and calcareous elastics during the Middle Jurassic. In the Oxfordian, a carbonate ramp was constructed, composed of in situ crusts and proximal breccias that indicate the largest extension of the Mazagan carbonate platform. The upper part of the Jurassic limestone sequence at Site 547 contains nodular limestones and pelagic wackestones, frequently inter layered with calcareous turbidites. The upward change in limestone composition suggests that the shallow-water platform edge retreated landward during the late Tithonian and Early Cretaceous as the Mazagan Escarpment was formed.

Jurassic Limestones of the Seaward Edge of the Mazagan Carbonate Platform, Northwest African Continental Margin, Morocco