Abstract Lithology is acknowledged to be an important internal catchment control on flow processes to adjacent alluvial fans. However, the role of inherited structural configurations (e.g. bedrock attitude) in catchment connectivity and sediment transport is rarely considered. We examine four young (<100-year-old) active tributary junction alluvial fan systems from the Dadès Valley in the High Atlas of Morocco in terms of their catchment-scale connectivity, sediment transfer and resulting alluvial fan processes. The catchments occur on the same lithologies (limestones and interbedded mudstones), but experience different passive structural configurations (tilted and structurally thickened beds). The fan systems react differently to historical peak discharges (20–172 m 3 s −1 ). Catchments containing tectonically thickened limestone units develop slot canyons, which compartmentalize the catchment by acting as barriers to sediment transfer, encouraging lower sediment to water flows on the fans. Syn-dip catchments boost connectivity and sediment delivery from translational bedrock landslides as a result of steep channel gradients, encouraging higher sediment to water flows. By contrast, translational landslides in strike-oriented drainages disrupt longitudinal connectivity by constricting the valley width, while the gradients of the main channels are supressed by the attitude of the limestone beds, encouraging localized backfilling. This diminishes the sediment to water content of the resulting flows.

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.