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Alonso Zarza, A.M., Calvo, J.P. and Garcla del Cura, M.A., 1992. Palustrine sedimentation and associated features-- grainification and pseudo-microkarst--in the ...
Sedimentary Geology, 76 (1992) 43-61

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Elsevier Science Publishers B.V., Amsterdam

Palustrine sedimentation and associated features grainification and pseudo-microkarst in the Middle Miocene (Intermediate Unit) of the Madrid Basin, Spain A.M. Alonso Zarza a j.p. Calvo b and M.A. Garcla del Cura a a U.E.I. Petrologfa, Inst. Geologfa Econdmica, C.S.I.C., Unicersidad Complutense, 28040 Madrid, Spain b Dept. Petrologfa y Geoqufmica, Facultad de C. Geoldgicas, Universidad Complutense, 28040 Madrid, Spain (Received May 22, 1991; revised version accepted October 9, 1991)

ABSTRACT Alonso Zarza, A.M., Calvo, J.P. and Garcla del Cura, M.A., 1992. Palustrine sedimentation and associated features-grainification and pseudo-microkarst--in the Middle Miocene (Intermediate Unit) of the Madrid Basin, Spain. Sediment. Geol., 76: 43-61. A rather thick sequence (50-75 m) of lacustrine limestones forms the uppermost part of the Intermediate Unit (Middle Miocene) in the NE part of the Madrid Basin, central Spain. A gradual change from distal alluvial to open lake facies is recorded in this area. Red mudstones passing vertically to strongly mottled limestones are typical of the transition from distal alluvial to marginal lake deposits. A wide variety of facies occur in the marginal lacustrine facies association, all of them showing pedogenic features (e.g. root traces, alveolar septal structures, pseudo-microkarst). Micrites and biomicrites are interpreted as open lacustrine deposits. Although not as strongly pedogenically modified as the marginal lacustrine facies, they still show evidence of subaerial exposure. The carbonate sequence was deposited in a low-gradient, shallow-lake system in which slight fluctuations in the water level caused extensive emergence. Pedogenic processes affected most of the lake deposits, but modification was more intense in the marginal facies. Biogenic activity and mechanical reworking at lake margins led to the formation of pedogenic-diagenetic pseudo-microkarst and peloidal a n d / o r intraclastic limestones.

Introduction

Palustrine carbonates constitute a characteristic and diagnostic set of lithofacies in many shallow-lake systems. They have been extensively described from both Mesozoic and Cenozoic continental basins of southern France, Spain, western U.S.A, and other countries (Freytet, 1973; Freytet and Plaziat, 1982; Mel6ndez, 1983; Wells, 1983; Cabrera et al., 1985; Arribas, 1986; Platt, 1989; among others). Palustrine facies are deposited in shallow-lake settings with low-gradient margins in which small-scale water-level fluctuations cause exposure of large areas (Platt and Wright, 1991). In terms of facies models, palustrine carbonates record sedimentation at low-gradient, low-energy "ramp"-type lake margins in the four-fold classi-

fication proposed by Wright (1990a) and Platt and Wright (1991), based on the morphology and energy of lake margins. Palustrine limestones show features both of the primary lacustrine carbonate and those derived from subsequent modification (organisms, root traces, desiccation marks, and other pedogenic fabrics) (Freytet, 1984). Physical and biological processes active in the palustrine environment account for a development of a variety of secondary diagenetic and pedogenic fabrics. Some characteristic features that have been commonly reported from palustrine sequences include: mottling, pedogenic nodules, brecciation (caused either by desiccation or root bioturbation) and reworking of carbonate fragments (Wells, 1983; Freytet, 1984; Platt, 1989).

0037-0738/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

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Other more complex features, including development of pseudo-microkarst cavities (Plaziat and Freytet, 1978; Freytet and Plaziat, 1982) and grainification (Mazzullo and Birdwell, 1989; Wright, 1990b), have also been described from palustrine carbonate sequences. In addition, the occurrence of evaporites (anhydrite, gypsum), dolomite, or other minerals has also been reported, reflecting higher salinities than those common in temperate "hardwater" carbonate lakes (e.g. marl lakes of Michigan; Murphy and Wilkinson, 1980). This paper deals with the sedimentological characteristics of Tertiary (Upper Miocene) lacustrine sequences in the northeastern part of the Madrid Basin, central Spain. In addition to describing the lacustrine carbonates and presenting an integrated facies model for their deposition, we discuss the formation of pseudo-microkarst features as well as the role of grainification in the formation of these palustrine deposits.

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Geological setting and sedimentary context The study area lies in the northeastern part of the Madrid Basin (Fig. 1), a large basin which extends over more than 10,000 k r n 2 in the interior of the Iberian Peninsula (Fig. 1). Throughout the Tertiary the basin was filled by terrestrial deposits, which exceed 3,500 m in thickness. During the Neogene three main tectono-sedimentary units (the Lower, Intermediate and Upper Miocene Units) were deposited in the basin (Calvo et al., 1989; Ordofiez et al., 1991). These units record a succession of alternating lake complexes and alluvial sedimentary systems; deposition took place according to a concentric facies pattern (Calvo et al., 1991). Figure 2 shows a general lithostratigraphic log of the sedimentary filling of the northeastern part of the basin throughout the Neogene. At the top of the Intermediate Unit, a major phase of lake expansion in the basin is recorded through the Upper Aragonian and Lower Vallesian (correlative with Upper Serravallian-Lower Tortonian marine stages). By this time, the lake system had expanded considerably, covering previously deposited alluvial formations so that the lacustrine

Fig. 1. Location map showing the position of the study area (rectangle) in the Madrid Basin.

sequence onlapped the basin margins in most of the area (Alonso Zarza et al., 1990). The lacustrine unit capping the Intermediate Miocene Unit is formed of a monotonous sequence of carbonates with minor intercalations of marls, clays and sandstones. The thickest lacustrine succession (75 m) occurs at Almadrones (Fig. 3), 10 km away from the basin margin. The lacustrine deposits thin progressively to the east and north away from this point. A general palaeogeographic model for the Miocene lacustrine unit studied in this paper is presented in Fig. 3. The model emphasizes the distribution pattern of the alluvial systems, which controlled the geometry of a shallow, generally hydrologically closed lake developed in the basin centre.

Sedimentology of the lacustrine carbonate system Lake deposits in the NE area of the Madrid Basin crop out in a rectangular area relatively

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types occurring in the area. These facies types were deposited in distinct, but closely related sedimentary subenvironments, each assigned to one of three facies associations: (a) distal alluvial-lake margin transition; (b) marginal lacustrine; and (c) open lake.

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Fig. 2. Idealized stratigraphic log of the Miocene in the N E of the Madrid Basin. The upper limestone sequence of the Intermediate Unit (from 240 to 290 m) is the subject of the paper.

close to the basin margins (Fig. 1). Towards both the northern and eastern margins, these lake deposits interfinger with fine red clastic sediments (distal alluvial fan facies). Towards the south and west, the lacustrine facies form a 50-75 m thick sequence of freshwater limestones with few intercalations of alluvial clastic facies (Fig. 4). The lacustrine facies have been analysed in nine stratigraphic sections. Figure 5 presents three selected logs showing all the lacustrine facies

The interfingering between distal alluvial and lake margin facies is observable in an irregular belt that runs roughly parallel to the basin margins. The geometry of this belt is controlled by and related to the location of the major alluvial systems. Thus, in areas where major alluvial systems drained into the basin, the transitional belt was located further from the basin margins than in areas where the alluvial systems were few or inactive (Alonso Zarza et al., 1992a). Successions are dominated by red and green mudstones a n d / o r marls and mottled limestones, except locally in the south where red mudstones, sandstone- and gravel-filled channels and nodular mottled limestones record active progradation of fluvial channels on the lake margins. Red mudstones represent the typical distal alluvial facies. They are formed mainly of illitic clays with minor smectites or illite-smectite mixed-layers. This facies includes root moulds (rhizoliths; Klappa, 1980) and carbonate nodules and usually shows green drab-haloes (Retallack, 1988). Towards the top, the red mudstones become progressively enriched in carbonate passing up into orange or red marls with prismatic structure overlain by massive or horizontal bedding at the top. This change can be easily observed at the base of the Castej6n de Henares (Fig. 3) section (Fig. 5-1). Green mudstones and~or marls occur in thinner beds (up to 0.5 m) at the top of the red mudstone facies. Their mineralogy is essentially the same as that of the red mudstones. They also include small (2-3 cm) carbonate nodules and root moulds. Mottled and nodular limestones outcrop as sheet-like beds averaging 2 m in thickness. These beds become thinner near to the basin margins, where they pass laterally into red marls a n d / o r

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mudstones. The limestones display a distinctive nodular structure and strong yellow-red mottling. The limestones are micritic with minor amounts

of fine detrital grains. Mottling is evident under the microscope as very diffuse reddened or blackened haloes. Root moulds, fenestral and alveolar

Fig. 4. General view of the lacustrine limestones overlying alluvial deposits. The sheet-like geometry_ of the beds can bc appreciated.

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structures are also common in these limestones, but they are not as well developed as in deposits of the marginal lacustrine facies association. Des-

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