Controls on Devonian hemi-pelagic limestone deposition analyzed on cephalopod ridge to slope sections, Eastern Anti-Atlas, Morocco

Abstract

The hemi-pelagic TaWlalt Ridge separating the Maider Basin from the TaWlalt Basin developed progressively from an Early Devonian homoclinal ramp, through a Middle Devonian ramp-slope stage of moderate topography, to a mature cephalopod ridge during the Late Devonian and formed a spur-like element extending from the southern shallow-water Maider Platform to the broader northern hemi-pelagic TaWlalt Platform. During the Middle Devonian, thick lowstand aprons were shed onto the ridgeslope from an active mid-ramp carbonate factory in the south (Maider Platform). The shallow-water derived sediments Wrst by-passed the central paleohigh, but then onlapped the ridge during start of sea-level rise. The allodapic limestones (storm-induced turbidites, tempestites, and debrites) of the aprons consist of large parts or entirely of reworked lithic peloids. The lithic peloids are interpreted as the main foundation of all micritic lithofacies types in the distal ramp locations, including those on the hemi-pelagic cephalopod ridge. Lateral facies transition of allodapic limestones into nodular cephalopod limestones on the ridge suggests the latter originate from allodapic beds and were subsequently transformed into nodular limestones by bioturbation, early diagenesis, and the faunal input from a hemi-pelagic community. Iron-rich hardgrounds formed on the ridge during major regressive phases as a result of increased winnowing by the wave-base and correlate with siliciclastic turbidite deposition on the slope. The hardgrounds can be correlated with hiatuses on the TaWlalt Platform, which formed a broader hemi-pelagic swell to the north. During the Late Devonian, the southern Maider Basin carbonate factory became unproductive or disappeared, and a ridge facies of truly hemi-pelagic autochthonous limestones developed. Allodapic deposits on the slope were shed from the ridge itself at this time but did not form well-deWned aprons.

Fossil‐Lagerstätten, palaeoecology and preservation of invertebrates and vertebrates from the Devonian in the eastern Anti‐Atlas, Morocco

“…Both thickness and facies variation increase through the Devonian (Hollard ; Wendt ; Kaufmann ; Döring & Kazmierczak ; Döring ; Lubeseder et al . ). The maximum thickness of the Middle Devonian sedimentary succession in this small basin can be found east of Tazoulait and reaches over 700 m (Fig.…”

“…The corresponding succession in the Tafilalt region is much thinner and varies mostly between 30 m on the Tafilalt Platform (e.g. at Bou Tchrafine, Hamar Laghdad or Filon Douze) and over 220 m at Ottara, which was probably situated on the slope towards the Maïder Basin (Kaufmann 1998;Lubeseder et al 2010). Throughout most of the eastern Anti-Atlas, the Middle Devonian succession is dominated by limestone, marl and claystone alternations.…”

“…Throughout most of the eastern Anti-Atlas, the Middle Devonian succession is dominated by limestone, marl and claystone alternations. The closer to the depocentre in the Maïder Basin, the greater gets the proportion of mudstones and the thicker the clay proportion of the succession (Kaufmann 1998;Lubeseder et al 2010). On the Tafilalt Platform, condensed sedimentation prevailed; cephalopod limestones (dacryoconarid wackestones and packstones) are common and macrofossils occur in great numbers (Wendt 1988;Kaufmann 1998;Döring & Kazmierczak 2001).…”

“…Fortunately, the arid climate keeps vegetation to a minimum in the Moroccan pre-Sahara, thus creating extensive outcrops. Therefore, thickness and facies changes can be examined in space and through geological time in order to reconstruct the arrangement of basins and carbonate ramps as well as the approximate course of coastlines throughout the Devonian (Hollard 1974;Wendt 1985Wendt , 1988Wendt & Belka 1991;Kaufmann 1998;Döring & Kazmierczak 2001;Lubeseder et al 2010). Since the required palaeogeographical data are readily available from the literature, we do not add new details to this aspect, but provide a brief review necessary for the interpretation of the Lagerstätten type.…”

Middle-Late Ordovician iron-rich nodules on Yangtze Platform, South China, and their palaeoenvironmental implications

“…Similar iron-bearing structures are widespread around the world at specific intervals from the Ordovician to the Holocene and can be roughly divided into three types according to the bathymetric position in which they developed: (1) shallow water type, which is developed close to land, usually associated with ooidal ironstone or ferruginous laminated structure (Sturesson 1989;Burkhalter 1995;Giresse et al 1998;Ferretti 2005;Ferretti et al 2012;Salama et al 2012Salama et al , 2013Salama et al , 2014Zato n et al 2012;Andrews et al 2015); (2) relatively deep-water type, which require condensation or sediment-starved conditions (F€ ursich et al 1992;Preat et al 2000Preat et al , 2008Mamet & Pr eat 2003;Gradzi nski et al 2004;Cavalazzi et al 2007;Lubeseder et al 2010;Reolid & Nieto 2010;McLaughlin et al 2012;Laz ar et al 2013;Laz ar & Gr adinaru 2014); and (3) deep-ocean type, which is mostly interpreted as a result of an extremely low sedimentation rates and/or seafloor erosion, coupled with gradual precipitation over thousands of years, although a few of them are associated with hydrothermal vents or cold seeps where they may have formed more rapidly (Barbieri et al 2004;Hein & Koschinsky 2014). Although opinions vary as to the origin of these structures, all of the structures are thought to have had a microbial origin or at least to have had microbes as an important agent in their formation.…”