Dynamic stratigraphy as a tool in economic mineral exploration: ultra-pure limestones (Upper Jurassic, SW Germany)

Abstract

Ultra-pure limestones form valuable economic minerals used in the glass, paper, pharmaceutical, chemical and building industries. Both exploration and exploitation of economic materials require predictions of the subsurface stratigraphy. Such predictions are most reliable when the depositional processes creating these deposits are well understood. We present a simple, process-based methology (‘Dynamic Stratigraphy’) to break down stratigraphic packages into a hierarchy of easily recognisable genetically defined units of various scales. Each scale of analysis at the same time provides critical data on the composition, quality, volumes and preferred occurrence of economic minerals. In this study, Upper Jurassic carbonates of SW-Germany, deposited on a deeper epicontinental ramp, have been re-evaluated, based on newly available borehole data of an exploration programme for industrial minerals, especially for ultra-pure limestones, carried out by the Geological Survey of Baden-Württemberg. About 2.5 km of core were slabbed and continuously studied for micro- and macrofacies, geochemistry, colourimetry (‘whiteness index’) and calibrated to the gamma-ray signatures. In a number of key outcrops stratal and facies patterns as well as sedimentary geometries were mapped. The carbonate buildups form the most promising rock bodies hosting ultra-pure limestones. Gamma-ray signatures correlate both with major facies types and their bulk chemical composition and are thus useful for an initial evaluation of raw materials properties. In both major facies associations, (1) the bedded and (2) the massive microbial/sponge buildup facies, distinct sedimentary cycles were recognised, building systematic stacking patterns and a cycle hierarchy. Within the buildup facies, this cyclicity mirrors the ‘whiteness’ and ‘pureness’ of limestones. The cycles are used to carry out regional correlations between and across extensive buildup complexes, which are generally regarded as completely massive and homogenous. Based on this, the occurrence and distribution of economically valuable ultra-pure limestone bodies can be understood, and used to develop predictive genetic models. Dynamic or genetic stratigraphy integrated with petrophysical measurements (colourimetry, gamma-ray logs) leads to more cost-effective techniques of exploration and exploitation. While similar sedimentological concepts are commonly used in the petroleum industry, this study documents their usefulness also in the economic minerals industry.