66-1 Persistence During Stressful Times: A New Record of Bajocian (Middle Jurassic) Reefs from the Central High Atlas Mountains of Morocco

Session: Environmental Instability During Greenhouse Periods: Impact on Terrestrial and Marine Ecosystems (Posters)

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The early Bajocian Propinquans Zone (~170.3 Ma) marks an interval of reduced carbonate accumulation in the Central High Atlas Basin of Morocco, characterized by widespread platform suppression and elevated nutrient and siliciclastic input. These conditions have been linked to climatic cooling, decreased  pCO₂, and enhanced runoff; however, pronounced spatial variability within the basin reveals that carbonate systems did not collapse uniformly. Localized reef development at Tidrite demonstrates that some environments remained conducive to coral reef growth despite regional suppression.

Field observations near Tidrite (Dadès Valley) document multiple coral-bearing patch reef horizons that contrast sharply with platform suppression elsewhere. Preliminary field and petrographic data from the Bin el Ouidane Group identify a reef-bearing interval along a gently sloping carbonate ramp, with reefs concentrated along the southern margin of the Msemrir minibasin. Reef formation coincides with a relative sea-level rise, where moderate deepening and increased accommodation space may have facilitated coral colonization.

Six coral reef horizons occur within ~120 m of stratigraphic section, spanning the Propinquans to lower Humphresianum zones. Buildups range from 1.3 to 4 m in height and 10 to 120 m in width, exhibiting both biostromal and biohermal geometries. Most feature small reef cores (1–3 m tall, <10 m wide) flanked by laterally extensive skeletal deposits. Reefs are underlain by skeletal rudstones (brachiopods and oysters), onlapped by oolitic grainstones, and capped by marls or ooid-rich facies. This stacking pattern suggests development within an upper offshore to lower shoreface setting, reflecting long-term deepening overprinted by shorter-term shifts in energy and sea level.

Petrographic and outcrop data show a clear upsection decline in microbialite abundance and an increase in coral framework dominance. The lowest reef is predominantly micritic matrices and bioturbated skeletal rudstones with <10% coral content. Microbial fabrics are clotted to stromatolitic, and coral forms are limited. Younger reefs show declining microbial textures and increasing coral abundance, including dense branching, domal, and platy forms. Associated fauna include encrusting dasycladacean algae, bivalves, brachiopods, serpulids, gastropods, echinoids, and foraminifera. These patterns suggest that localized paleoenvironmental factors controlled reef development at Tidrite. Rather than a basin-wide shutdown, Tidrite represents a reefal refugium during early Bajocian environmental stress.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9974

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