Abstract The well-exposed Djebel Bou Dahar (DBD) carbonate platform (Lower Jurassic, High Atlas, Morocco) demonstrates the role played by different carbonate factories on the growth and architectural evolution of a high-relief, flat-topped carbonate depositional system. It also shows, in contrast with the generally accepted idea that lithiotid bivalve accumulations dominated Lower Jurassic platform margins, that microbial carbonates substantially contributed to the carbonate factory, as in Upper Jurassic reefs. The DBD carbonate depositional system accumulated on the footwall high of an active marine rift. Its depositional architecture evolved from a low- relief ramp profile (Hettangian p.p.-Sinemurian) to a high-relief platform with slopes up to 32° and 600 m in relief (uppermost Sinemurian- Pliensbachian) as a function of changes in accommodation and carbonate factory. The Sinemurian low-relief system consisted of siliceous sponge microbial mounds associated with coated grain skeletal packstone and grainstone in middle and outer ramp facies belts. This deep-water carbonate factory did not build into wave-agitated shallow settings and lacked the capability of constructing high-relief platform margins. From the latest Sinemurian, the platform built significant relief and the slope steepened (20-32°). This switch in platform architecture coincided with the accumulation of a highly productive, coral calcareous sponge microbial boundstone at the platform margin and on the slope (from 10 to 60 m in depth). This was adjacent to deeper water siliceous sponge microbial boundstone (from 60 to 140 m below the platform break). During the late Pliensbachian increased accommodation and retrogradation, coral calcareous sponge microbial boundstone extended from the upper slope onto the outermost platform, 350 to 400 m inward of the platform break, associated with microbial microencruster boundstone and lithiotid bivalve biostromes. During this aggradational- retrogradational phase, microbialites were able to expand on the outer platform top because low-energy substrates were made available on the platform top by increased accommodation. Outer platform strata consisted of coral calcareous sponge microbial boundstone and coated grain skeletal grainstone, dipping 5° basinward, as observed in other Mesozoic and Paleozoic microbial boundstone-dominated platform margins. The platform interior was dominated by subtidal peloidal skeletal packstone with Cayeuxia-calcified cyanobacteria and intertidal fenestral packstone with laminated microbial boundstone, which contributed to the sediment budget maintaining a flat-topped platform interior geometry. The DBD shares similarities for facies and depositional geometry with Upper Jurassic Southern Tethyan and North Atlantic carbonate systems, implying that the main components of Upper Jurassic reefs were already present in the Early Jurassic rift basin of Morocco.

Abstract The Middle Devonian carbonates of the Slave Point Formation, Hamburg field, northwestern Alberta, are composed mainly of stromatoporoid and Amphipora floatstones and rudstones, with interbedded mudstone and grainstone facies characteristic of deposition in open to slightly restricted marine platform environments. These carbonates have undergone a complex diagenetic history, from shallow to deep burial, as represented by fracturing, calcite cementation, silicification, and dolomitization. Petrographically, four different types of dolomite have been identified (from early to late): (1) fine-crystalline matrix dolomite; (2) pseudomorphic dolomite; (3) medium-crystalline pervasive dolomite; and (4) saddle dolomite. Fine-crystalline dolomite (5–50 (μm) replaces the mud matrix and slightly penetrates the edges of allochems. It occurred in mud-supported facies and was precipitated by marine fluids. Oxygen isotope values range from −11.62 to −9.34‰ (Peedee belemnite), lower than postulated values for Devonian carbonates. The enriched 87 Sr/ 86 Sr isotope value from this phase (0.71002) suggests that later diagenetic fluids may have recrystallized this dolomite. Pseudomorphic dolomite (50–100 μm) replaces crinoids and occurs as single, large dolomite crystals. Its oxygen and carbon isotopic values range from −10.58 to −9.65 and +4.24 to +4.49‰, respectively. Medium-crystalline pervasive dolomite (10–100 μm) occurs along dissolution seams and obliterates all previous fabrics. It is proposed that this medium-crystalline dolomite formed during shallow to intermediate burial because of its association with dissolution seams and high iron content. The range of oxygen isotope values for this dolomite (−11.74 to −9.5‰) suggests precipitation from a warm fluid, possibly in a burial environment, and/or later recrystallization by hydrothermal fluids. The relatively wide range of carbon isotope values (+1.19 to +4.49‰) and enriched strontium isotope ratio (0.710020) suggests recrystallization. Saddle dolomite (250–2000 μm) partially to completely occludes void spaces (both fractures and vugs) and also occurs as a minor replacement mineral. The oxygen isotope values for saddle dolomite (−?13.95 to −?11.97‰), as well as the nonradiogenic to enriched strontium isotope ratios for saddle dolomite (0.70494 to 0.710351), and the fluid-inclusion data (homogenization temperature, T h , range between 125 and 161°C and estimated salinity, between 22.2 and 24.7 wt.% NaCl equivalent) indicate precipitation from hot, highly saline, hydrothermal fluids, which were probably expelled tectonically during the Late Devonian-Mississippian Antler thrust belt development.