X-Ray Mineralogy of Rectorite-Containing Lower Devonian Marine Sediments, at Jebel Issimour, Anti-Atlas, Morocco

Quantitative representations of whole sample and clay fraction mineral content were obtained by XRD techniques from 34 samples of a 130 m thick section of Lower Devonian siliciclastic and carbonate sediments from the Lochkovian into the Upper Emsian, exposed in Jebel Issimour, Anti-Atlas, Morocco. The goals of the investigation were to determine whether the mineral content could be correlated with magnetic susceptibility (MS) measurements and to assess whether the increased abundance of land plants during this time interval produced recognizable changes in the clay assemblages being contributed to this shallow marine site. Total quartz and calcite averages about 80 wt% and their individual abundances are inversely correlated. In the more quartz-rich sediments, clay minerals (up to 40 wt%) and trace quantities (<5 wt%) of plagioclase, K-feldspar, and pyrite are present. Minor quantities (<10 wt%) of dolomite and anhydrite were present in the calcite-rich sediments. Cyclic increases in quartz content are visually correlated with decreased MS. A prominent negative excursion in MS during the Lower Emsian is coincident with the most quartz-rich sediments. The three major minerals in the clay fraction are 2M1 illite with a low Fe content, IIb Fe-rich chlorite (chamosite), and a regularly interstratified illite/smectite (rectorite). Rectorite is abundant in samples from the Lochkovian, and occurs as a minor constituent in most of the overlying sediments. The rectorite-illite-chlorite assemblage is interpreted as a burial diagenesis sequence formed from detrital materials that were initially richer in smectite. Smectite layers were converted to an illite/smectite plus chlorite during burial. There are no obvious associations of clay mineral changes with the increased abundance of land plants.