Abstract

Several paleokarst surfaces are recognized within the Neogene sedimentary record of the Madrid Basin, central Spain. During the Late Vallesian, one of these paleokarsts was related to a major break in the Miocene sedimentary sequence. The paleokarst developed diachronically over fresh-water carbonates in central and eastern parts of the basin. A variety of both exokarstic and endokarstic morphologies and associated sediments (breccias, siliciclastic infill deposits, speleothems) are distinguished. The paleokarst records a complex history of dissolution, collapse and infilling processes that were related to long-term subaerial exposure of carbonate. In this chapter, emphasis is placed on paleotectonic constraints leading to the formation of paleokarst. Paleokarst development was related to a drastic change from compressive to extensional strains in the area during the Late Vallesian.

Introduction

As stated by Wright (1991a), ‘paleokarst refers to karstic features formed in the past, related to an earlier hydrological system or landscape’. Most specific karstic features deal with small-to large-scale dissolution processes that affect highly soluble rocks, mainly carbonates and evaporites, and lead to well-developed secondary porosity (Ford & Williams, 1989; Smart & Whitaker, 1991). The importance of paleokarst as a relevant aspect in stratigraphic analysis or in exploration strategies is reflected in a rather abundant recent literature on this topic (Wright, 1982; Esteban & Klappa, 1983; James & Choquette, l988; Bosak et al., 1989; Wright, 1991b).

Most paleokarsts recognized in the sedimentary record can be attributed to meteoric waters, as most of them show morphological patterns similar to recent meteoric karst (Ford, 1988; Ford & Williams, 1989).