PALEOCLIMATIC IMPLICATIONS OF MID- TO UPPER HOLOCENE CALCAREOUS TUFA DEPOSITS FROM THE BOURKAÏZ ALLUVIAL FAN, NORTHERN MOROCCO

This study deals with the calcareous tufa deposits of the Sais Basin of north-central Morocco. This basin was a largely marine basin during late Neogene time, but by latest Neogene (late Pliocene) time, the basin began infilling with alluvial-lacustrine deposits. By Mid-Holocene time, however, parts of the basin began developing calcareous tufa deposits related to the dissolution of older limestones. The deposits occur as barrages (dams) up to a meter high, fluvial-lacustrine channel deposits of calcareous sand and oncoids, and fluvial-palustrine deposits of dark, organic-rich, clay and carbonate muds. The carbonates include calcite, aragonite and dolomite, and SEM analyses indicate the presence of microbial algal filaments along with biota including charophytes, rhizoliths, gastropods, and ostracods. Their inclusion within the tufas suggests their involvement in the organo-mineralization of the calcareous tufas.

Using the plots of Gandin and Capezzuoli (2008) and the equations of Tongiorgi and Panichi (1976), carbon-isotopic signatures, ranging from -10.2 to -6.7 ‰VPDB, support the interaction of organic matter and the biogenic origin of CO₂ in tufa formation. On the other hand, oxygen-isotope signatures, ranging from -8.4 to -4.8 ‰ VPDB, reflect the importance of intense evaporation in carbonate formation. Moreover, radiocarbon dating of four samples from the fluvial-lacustrine facies shows ages ranging from 6 to 2 Ka (calibrated B.P.). The older two dates reflect deposition during the African Humid Period (AHP), a pluvial period during which high-energy floods dominated in the study area, whereas the younger two dates represent later deposition during Roman-Iberian Climate Optimum, a more stable, warm, evaporative period across the Mediterranean area, during which the slower deposition of organic-rich, muddy carbonates predominated. An unrecognized break in deposition or diastem may exist between older and younger deposits. The more modern geochemical methods used in this study provide important supplemental information about paleoclimate contributing to the deposition of calcareous tufas on an alluvial fan.