Nuevas aportaciones al conocimiento geológico de la Cuenca de Madrid

Abstract

The Madrid Basin, now extended over more than 10.000 square km in the center of the Iberian península, is made up by a thick infilling of Tertiary sediments, ranging from Eocene to Pliocene in age. Outcrops of these sediments (neogene series exposed in the central parts, paleogene series in the marginal areas of the basin), along with drilling data as well as seismic profiles have permited to the spanish researchers to build up the stratigraphy of this sedimentary setting. The tectosedimentary analysis (Megías, 1982) is used paper as a practical basis for the interpretation of the continental basin of Madrid. This technique aims to divide the sedimentary filling into tridimensional bodies with their own specific features ("sedimentary ruptures"). A tectosedimentary unit is defined as a stratigraphic unit made up by a succession of strata deposited in a geologic time interval and controlled by related tectonic and sedimentary parameters. A tipology of the geometric relationships of these units is shown (Fig. 1). Their comparison with other kinds of stratigraphic units also discussed. On the basis of this analytical method several main tectosedimentary units have been distinguished in the Neogene in the cental part of the basin. Ruptures limiting these units can be matched with their correlatives in the marginal areas. "Saline Unit": Our knowledge in this unit is mainly supported by drilling data obtained from the exploration of sodic salts. The upper 300 meters are composed of anhydrite-magnesite (and dolomite) alternaces, with shaly and glauberite beds. Thenardite-bearing sediments in the upper part of the serie may be envisaged as a sub-unit of that Saline TSU. Evaporite phases and textural and structural evidences lead us to suppose that the Saline Unit was principally deposited in a perennial lake where brines underwent freshening processes. "Detrital-Gypsiferous Unit": Field appearances (regular lamination, detrital texture and sedimentological features) allow us to conclude that detrital-gypsum sediments of these units were mainly deposited as gravity-flow (mass-flow, grain-flow, turbidite,...) deposits in a saturated, fairly deep lacustrine basin. Pink-light evaporites beeds, more frequent in the upper unit, are envisaged as "chemical" deposition during non-detrital episodes. "Terminal Fluvio-lacustrine Units": Three units with alluvial-to-lacustrine characteristics are distinguished in the uppermost section of the Neogene serie: a) Lower fluvio-lacustrine unit, mainly compposed of carbonate sediments deposited in shallow lacustrine envirenments. These carbonates were affected by intensive diagenetic processes (dedolomitization, recrystallization, silicification), partly related to a subsequent episode of karstification. b) Intermediate fluvio-lacustrine unit, constituted by basal fluvial deposits (quartzite clasts in the eastern side of the basin; arkosic sandstones with "granite" clasts in the western side). Limestones overlying these terrigenous beds were deposited in a fairly wide spectrum of "lacustrine" environments. c) Upper fluvio-lacustrine unit: mainly exposed at the Ocaña-Tarancón area, this unit is og Pliocene age. Fluvial deposits in this unit show a great content in biogenic carbonates (oncolites, stromatolites, bioclasts). A thick laminar calcrete overlies these deposits. Arkoses: Large deposits of arkosic sandstone outcrop in the NW and W of the Madrid Basin. Geometric relationship with other units are discussed. Having in mind the spatial distribution of two sepiolite beds quarried in some places of the basin we conclude the erosive disposition of the so-named "Arcosas de Madrid" over the central and intermediate units of the basin. The hipothesis of the correlation between this arkosic formation and the Intermediate fluvio-lacustrine unit is mantained.