Palaeogeographic controls on palygorskite occurrence in mid-Cretaceous sediments of Morocco and adjacent basins

Abstract

Palygorskite has been identified as a major constituent of the clay fraction in midCretaceous deposits of the Meseta, the Middle Atlas, and the Rif in central and northern Morocco. Two types of palygorskite are differentiated, based on the morphology of crystallites, bio-and lithofacies associations of the containing sediments. Type 1 occurs in sections of the Meseta and the Middle Atlas, where the mineral displays bundles of long fibres under the electron microscope and is associated with shallow marine carbonate and evaporite facies. Type 2 was found in the Rif, where palygorskite occurs mostly as bundles of shorter fibres in turbiditic sediments of a supposed deepmarine environment. Associated microfossils indicate reworking of the palygorskite-bearing sediment from shallower parts of the basin. Palygorskite apparently grew authigenically in the chemically restricted environments of the Meseta and the Middle Atlas, whereas it is of a detrital origin in the Rif section. We consider that sediments containing authigenically formed palygorskite (type 1) could be the source of detrital palygorskite (type 2). Type 1 deposits on the North African shelf were probably partly reworked and responsible for the supply of palygorskite to midCretaceous Atlantic basins, where the mineral is well known from several DSDP sites. As in the case of various Recent deep-sea palygorskite occurrences, aeolian transport may have significantly contributed to palygorskite supply to the ocean basins. RESUME: L'6tude de la min6ralogie des argiles des s6diments albo-c6nomaniens du Rif, de la Meseta et du Moyen-Atlas (Maroc) montre la pr6sence fr6quente de fortes proportions de palygorskite. Dans la Meseta et le Moyen-Atlas, ce min6ral s'est form6 sur place dans des milieux chimiquement confin6s (origine autochtone = type 1). En revanche, dans le Rif, la palygorskite associ6e ~ des d6p6ts gravitaires semble remani6e depuis des milieux de plates-formes peu profonds (origine allochtone = type 2). Sous des conditions climatiques favorables, la pr6sence de min6raux fibreux dans les bassins marocains parai't principalement contr616e par les pal6omorphologies continentales, l'activit6 tectonique des marges, les vitesses d'6rosion et s6dimentation, et le confinement plus ou moins prononc6 des environnements de d6pft. Les nouvelles donn6es pr6sent6es montrent que les palygorskites des bassins c6tiers marocains constituent une des sources potentielles des palygorskites oc6aniques qui, comme dans diverses s6ries r6centes, r6sultent parfois d'un transport 6olien.Palygorskite, a fibrous Mg-rich clay mineral, is often considered to be easily destroyed by transportation (e.g. as stated by Millot, 1970). It is, therefore, still widely held that palygorskite occurrences represent sites of authigenic formation of this mineral and that detrital accumulation is 9 1996 The Mineralogical Society

Pedogenesis and clay mineralogical investigation of soils formed on gypsiferous and calcareous materials, on a transect, southwestern Iran

“…Whereas moderate quantities of palygorskite were found in the two older formations, only small amounts were observed in the youngest formation. The association of smectite and palygorskite in the Oligo-Miocene limestone and marl is consistent with the results of others (Gindy et al, 1985;Shadfan et al, 1985a,b,c;Shadfan and Mashhady, 1985;Ingles and Anadon, 1991;Pletsch et al, 1996;Khormali and Abtahi, 2003). Khademi and Mermut (1998) found palygorskite and sepiolite and appreciable amounts of mica and smectite in the clay fraction of the Oligo-Miocene limestone of central Iran.…”

The Mg isotopic composition of Cenozoic seawater – evidence for a link between Mg-clays, seawater Mg/Ca, and climate

“…This is equivalent to a 17-40% increase in Mg content for an average shale (2.5 to 3.5 wt% MgO; Gromet et al, 1984). Evidence for an enhanced Mg sink in authigenic marine clays in the Cretaceous and Paleogene comes from peaks in both shallow and deep-sea sediments in the abundance of authigenic palygorskite, a clay mineral that contains up to 12 wt% Mg and whose appearance in deep-sea marine sediments has been attributed to warmer bottom water temperatures (Couture, 1977;Pletsch et al, 1996;Thiry and Pletsch, 2011).…”

Varied crystallization conditions for Neogene sepiolite and associated Mg-clays from Madrid Basin (Spain) traced by oxygen and hydrogen isotope geochemistry

“…It occurs in varied natural environments from soils of arid regions (Bachman and Machette, 1977;Watts, 1980), to peri-marine and marine sedimentary environments (Bowles et al, 1971;Galán and Ferrero, 1982;Weaver, 1984;Pletsch et al, 1996). However, it has been mostly described as crystallizing directly from solutions in lacustrine environments (e.g., Galán and Pozo, 2011, and references therein).…”