Macalester College Geology Department

Paleosols as Indicators of Paleoclimate in the Upper Cretaceous Maevarano Formation, Mahajanga Basin, Northwestern Madagascar.

The ~80m thick Masorobe Member of the Maevarano Formation preserves abundant paleosol profiles that are characteristically red (typically 10R 5/6) with drab gray to gray-green mottling that tracks sub-centimeter-scale root casts surrounded by 1-2-cm-thick reduction halos (typically 5GY 8/1). Root mottling in the Masorobe paleosols is predominantly vertical in orientation, with some root casts extending up to 50 cm down-profile. Pedogenic carbonate in the form of coalesced nodules and/or carbonate-encrusted rhizoliths occur in 36% of the sampled paleosols (13 out of 36 units display macroscopic carbonate accumulations), and is most abundant in the upper two-thirds of the Masorobe section. Paleosols of the Masorobe Member lack clearly defined peds or grain-coating argillans, and the minimal development of these features suggest that these paleosols are relatively immature. XRD analyses confirm that smectitic clays dominate throughout the Masorobe Member. Geochemical analyses (XRF) of two selected paleosol profiles reveal zones of accumulation of mobile elements such as Na and K (B horizons), as well as accumulations of Al (coupled with relative decreases in Si) that are interpreted to reflect horizons of clay accumulation (Bt horizons). One 2.25-m thick composite paleosol shows three discrete zones of accumulation.

Paleosols of the Masorobe Member can be classified as either Protosols or Calcisols, depending on carbonate content. Data indicate that these paleosols developed in a well-drained, semi-arid setting where water was present in sufficient quantity to mobilize soluble elements but not entirely leach them from the system. The presence of nodular carbonate horizons >15 cm thick suggest that the mean annual precipitation on the Masorobe alluvial plain was probably less than 760 mm. This paleoclimatic reconstruction is consistent with other aspects of the sedimentology and taphonomy of the Maevarano Formation.