Abstract

The recognition of biogenic features is not simple in ancient calcretes due to the overlapping of subsequent diagenetic processes. Laminar calcretes developed in Vertisols of the Marília Formation (Upper Cretaceous, Brazil) present an opportunity to recognize biogenic features and understand the role of organisms in their genesis and distribution. Slickensides, wedge-shaped peds, rhizoliths and redoximorphic features, and sheets and stringers of calcite are the most common characteristics. Based on macrofeatures and microfeatures distributed in the profile, habits and paragenetic association, and luminescence behavior in cathodoluminescence analysis, calcite occurrences were divided into: (1) recrystallized Microcodium, (2) recrystallized spherulites, and (3) grain-coating cements, nodules, and fracture fillings. The Vertisol profile consists of well-developed, poorly drained paleosol formed on floodplain deposits, in the medial part of the distributive fluvial system. The alternating availability of water in a paleoclimate with well-defined seasons promoted the carbonate accumulation in the Vertisol horizons. The main biotic mechanisms responsible for the carbonate distribution were related to plant root activity associated with microorganisms in the rhizosphere and invertebrate soil fauna, generating Microcodium as a product of intracellular root processes, and spherulites as a product of organic matter decomposition mediated by microorganisms. The proposed model supports the hypothesis that the calcification of horizontal root systems is the major contributor to the genesis of ancient laminar calcretes.

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