A survey of multi-stage diagenesis and dolomitization of Jurassic limestones along a regional shelf-to-basin transect in the Ziz Valley, Central High Atlas Mountains, Morocco

Abstract

Abstract Petrographic and geochemical data from five localities in the Ziz Valley of Morocco indicate that Jurassic limestones have undergone early diagenesis that varied with location from shelf to basinal settings, burial diagenesis that was most pronounced in basinal settings, and late diagenesis caused by compression and uplift of the High Atlas Mountains. Marine cements occur at all five localities from shelf-to-basin center, although cement types vary from peloidal microcrystalline cements updip on the shelf-to-equant calcite in basinal settings. Presence of moldic grains and/or Mg-poor, Fe-poor blocky cements suggest that meteoric waters influenced early diagenesis at all shelf localities and on an upturned fault block in the basinal region, leaving only one locality unaffected by early meteoric processes. 87 Sr/ 86 Sr ratios of 0.70810–0.70895 (greater than 87 Sr/ 86 Sr of coeval limestones), Mg contents that decrease upward from 47.5 to 43.0 mol% MgCO 3 , presence of dolomitized marine cements, and dolomite cements that postdate marine cements but predate meteoric-to-burial cements suggest that dolomitization and dolomite cementation at two shelf localities took place in mixed meteoric and marine waters early in diagenesis. However, poorer preservation of depositional fabrics, lower δ 18 O values, and larger and more anhedral crystals suggest that dolostones downdip underwent later modification during burial, whereas those updip did not. Compaction during diagenesis generated numerous concavo–convex and sutured intergranular contacts at updip shelf, downdip shelf, and basinal localities where earlier meteoric cementation was not extensive. Compaction was insignificant in more extensively cemented mid-shelf settings. High Sr (1200–3800 ppm) and Fe (1000–2300 ppm) contents in brachiopod grains suggest that LMC components underwent some modification during burial in basinal settings in Sr-rich reducing waters. Fe contents of late intergranular cements increase from 2000 ppm at the basin's edge to as much as 6000 ppm in the basin's center. Bedding-parallel stylolites occur at all localities. The most negative δ 18 O values of sparry dolomites near the Tizi n'Firest fault (−6.2‰ vs. PDB) imply diagenetic temperatures of 65–85°C assuming water δ 18 O values of 0.0–2.0‰ vs. SMOW. Those temperatures are much less than previous estimates of burial temperatures in the High Atlas basin. An isotopic gradient extrapolating to roughly 5‰/km in diagenetically modified dolostones likewise suggests a geothermal gradient less than gradients previously proposed for at least parts of the area. Comparison of morphologies of transverse stylolites, which are found at all localities, with morphologies of bedding parallel stylolites suggests that transverse stylolites formed due to compression during late diagenesis. Uplift accompanying that compression allowed influx of low-Mg waters that, along with other factors, caused calcitization of dolomites. The Fe concentration of calcite that fills late fractures increases from less than 2000 ppm at the basin center to values in excess of 3000 ppm at the basin edge, opposite trends in earlier cements and reflecting uplift of the High Atlas Mountains and resultant changes in patterns of groundwater flow.