CAVE SEDIMENTS AS INDICATORS OF GEOMORPHIC DEVELOPMENT, MATIENZO, NORTHERN SPAIN.

ANDY QUIN (Department of Geography Lancaster University) & Dr. David L. Higgitt (Department of Geography University of Durham)

ABSTRACT:

Abundant quantities of allogenic clastic sediments are often preserved within cave systems. Geomorphic and palaeoenvironmental data can be obtained from these deposits in a similar manner to those recorded from other sediment sinks. Sediment cores have been collected from several caves in the Matienzo region of northern Spain. Magnetic susceptibility profiles have been obtained for these samples in order to begin a reconstruction of the karstic evolution of the area. Four groups of caves are distinguished by their variation in susceptibility values. Provisional findings and discussions are presented along with planned future investigations.

INTRODUCTION:

The Matienzo karst is located in the northern Spanish province of Cantabria. The region presents several examples of well developed karstic features including the 26km2 karst depression (PLATE 1), after which the area is named and the numerous extensive cave systems that are found beneath the surrounding hillslopes. The depression appears to have evolved under assiduous karstic processes over an estimated minimum of 1.8 million years (Waltham, 1981.). Over one thousand cave entrances have been located and mapped in the area the longest of which, La Sistema de los Cuatro Valles, being over 41km long. The principal caves are shown in (FIGURE 2). Many of the caves are phreatic in origin, (PLATE 2), and development has occurred at several discrete levels, (FIGURE 3).

Cores of allogenic clastic sediments deposited within caves are collected and investigated in a broadly similar manner to which lacustrine, reservoir and marine sedimentary environments have been utilised to reconstruct past geomorphic and palaeoenvironmental events. Care was taken to avoid sites with possible autogenic inputs. Twenty sites from eleven separate caves have been sampled to date (FIGURE 2). The sample site in Cueva Regaton, part of the Sistema de Cubio-Regaton, is pictured in (PLATE 3). In particular magnetic susceptibility profiles are obtained for sediment profiles. Combined with cave survey data these data are used to fingerprint likely sediment sources and distinguish periods of geomorphic activity and processes.

The usefulness of cave sediments in this type of approach has been emphasised by several workers; Lowe and Gunn (1994), Ford and Williams(1989) point to the archaeological and palaeoenvironmental significance of cave sediments to explain this point. Bull(1977) highlights the isolation of cave sediments, within a cave environment, in his assessment of their usefulness in determining palaeoclimatic conditions. The relatively stable cave environment protects sediments from normal weathering processes. Conversely the relative isolation of cave sediments can lead to logistical problems in sample collection, sample sites can be several hours caving from a cave entrance. The complex nature of the sedimentary processes acting within a cave environment and highly variable lithologies can also make the interpretation of these deposits problematic, Osborne (1984) and Jennings (1985).

This poster reports on the findings of the initial year of a post graduate research project and as such only preliminary findings and conclusions can be advanced. More extensive sampling is envisaged combined with the utilisation of wider analytical techniques, dual frequency magnetic susceptibility, SEM, XRD and geochemical analysis to aid in the sediment provenance study and geomorphic reconstruction of the region.

SUSCEPTIBILITY VALUES:

Magnetic susceptibility profiles were obtained for the twenty sample sites using a Bartington MS2 susceptibility meter and MS2C sensor (for volume susceptibility measurements k). These profiles are displayed graphically in figure 4. The k values recorded fall within the range expected for clay minerals. Within the cores however four groups of caves can be distinguished in terms of the maximum susceptibility recorded in each profile. Group 1, Cueva Regaton, has the highest recorded susceptibility at 41.2. Group 2 with maximum k values ranging between 11.2 and 5.3 contains the following caves: Cueva del Arenal, Cueva Commellante, Cueva Torcida and Cueva de Fresnado 2. Group 3, Cueva de Coberruyo, Candenosa, Cueva de Rascavieja and Cueva del Regato, have relatively low maximum k values falling in the range 3.9 to 2.9. The lowest susceptibility values, 1.8 and 1.6, were recorded for the two caves Torca de Coteron (part of La Sistema de La Vega) and Cueva de Mostajo and form the group 4 caves. The sites from each group are colour coded, in figure 2.

GEOMORPHIC INTERPRETATION:

An earlier attempt to describe the karstic evolution of the Matienzo depression, (Waltham, 1981), lacked the fuller understanding of the extent of cave development that more recent explorations have revealed. This is particularly evident in the attempt to reconstruct palaeodrainage patterns.

It can be argued that caves with similar susceptibility values for their suites of sediments share a common morphogenic agent or are at least contemporaneous. Similarities in the physical characteristics of the caves within the four groups identified above support this assertion. Regaton, alone in group 1, is also unusual for the area in that it has considerable dry horizontal development at a relatively low altitude (180m). The caves of group 2, Arenal, Comellante, Torcida and Fresnado all contain at least seasonal active streamways. Coberruyo, Candenosa, Rascavieja and Regato appear to be short, high level fragments of once more extensive systems. The final association of caves in group 4, Coteron and Mostajo, are known to have the most extensive horizontal passage development of any of the areas caves. These large abandoned phreatic tunnels, lying between 230m and 240m altitude, contain large quantities of clastic sediments and often have extensive calcite and gypsum deposits.

The distinctive nature of the sediments and the caves within each group leads me to propose an outline of the karstic evolution of the area. These groupings suggest that at least four main phases of karstic evolution have occurred. The most continuous of these episodes coinciding with the formation of the extensive cave systems found in the group four caves, Coteron and Mostajo. More recent phases of development are represented by the group 1 cave, Regaton, and the present day active phase by group 2 caves, Arenal, Comellante, Torcida and Fresnado. An older episode being reflected in the higher level group 3 caves.

ACKNOWLEDGMENTS:

The help of the Matienzo Caving Expedition in particular Mr. Juan Corrin and Mr. Peter Eagan and the Cartographic Unit, Department of Geography at Lancaster University was invaluable in the production of this poster.

BIBLIOGRAPHY

BULL, P.A. (1977) 'Cave Sediment Studies In South Wales.' Studies In Speleology Vol.3 Part 1. pp. 13-24.' William Pengelly Cave Studies Trust Ltd.

FORD, D.C. & WILLIAMS, P. (1989) 'Karst Geomorphology And Hydrology.' Unwin Hyman Ltd.

JENNINGS, J.J. (1985) 'Karst Geomorphology.' Basil Blackwell Ltd.

LOWE, D.J. & GUNN, J. (1994) 'Conference Abstracts: Changing Karst Environments: Hydrogeology, Geomorphology and Conservation.' Cave and Karst Science, Transactions of the British Cave Research Association. Vol.21 No.1 pp. i.

OSBORNE, R.A.L. (1984) 'Lateral Facies Changes, Unconformities and Stratigraphic Reversals: Their Significance For Cave Sediment Stratigraphy.' Cave Science Vol. 11 No. 3. pp. 175 - 184.

WALTHAM, A.C. (1981) 'THE Karstic Evolution Of The Matienzo Depression, Spain.' Zeitschrift fur Geomorphologie Vol25,3 pp.300-312.

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