OUCC Proceedings 11 (1983)

Steven Gale

The entrance to Cueva del Osu (43° 15` 46" N, 1° 17` 15" W with reference to the Madrid meridian) is located at an altitude of 1230 m on the shoulder of Sierra del Brazu, a ridge between the valleys of the Riega el Brazu and the Rio del Texu. Both streams flow into Lago de la Ercina at an altitude of 1108 m (Frontispiece). The cave has been known for some time and was fully surveyed by OUCC in 1979 (Singleton and Laverty, 1979).

The entrance series of Cueva del Osu consists of a complex, essentially phreatic network, which must have developed beneath a local phreatic level of at least 1243 m (Figure 1). Since the height of the phreatic level within the limestone is ultimately governed by the height of the resurgence levels where the water leaves the aquifer, and since these resurgence levels are controlled by local base-level, in this case probably that of the streams incising across the limestone, then the levels of the valley floors must have been considerably above those of the present during the time of formation of the entrance series.

The passages of the entrance series have subsequently experienced breakdown followed by stalagmite precipitation, although the fractured nature of some of the stalagmite indicates that at least some post-stalagmite collapse has occurred. Neither process appears to be active at present in this part of the cave.

Further into the cave, the entrance series has been deeply incised by vadose flow, with the consequence that the present route on descends steeply along vadose trenches and down several climbs, including a 30 m pitch, to an altitude of approximately 1165 m. The original phreatic morphology of the passage can be traced in the roof of the vadose trench as far as the top of the 30 m pitch. Beyond that point, however, roof collapse has destroyed any evidence of the original phreatic passage, and its former route can only be conjectured. The adjacent cave of Cueva de la Caña (Singleton and Thwaites, 1979) appears to have experienced a similar episode of vadose incision in passages located at almost the same altitude as those in Cueva del Osu (Figure 1). This may indicate its development as part of the same system.

Below the various pitches and climbs, the vadose passage appears to form a discrete element, and the original phreatic cave, if it exists at all, must be presumed to continue at a higher level. This part of the cave consists of an abandoned streamway within which the remains of stalagmite false-floors and fluvial sediment fill testify to the occurrence of several episodes of stream flow and possible stream abandonment during its history. Interestingly, there is little indication of diagnostically phreatic forms in the roof of the passage, and it must be assumed that the phreatic phase of passage development was rather short-lived, and that the cave was largely the result of rapid incision by vadose flow.

Continuing down the cave, the vadose passage joins the present active streamway at the T Junction (Figure 1). The phreatic origin of the main streamway is clearly indicated by avens of up to 0.5 m diameter and scallops within the passage roof. Since the tributary passage is vadose down to its junction with the main streamway and appears to have developed in response to the formation of the main streamway under phreatic conditions, then the phreatic development of the streamway itself must have taken place under a local phreatic level of approximately 1165 m. Other vadose inlets to the main streamway, which may have developed synchronously with that of the abandoned vadose passage, include Stone-Lid Cave (Singleton and Thwaites, 1979) and possibly Pozu las Nieves (Singleton and Laverty, 1979, Fig.l). All these factors indicate that by this time local phreatic levels must have fallen by at least 78 m since the formation of the entrance-series passages. The morphological evidence in the cave suggests that this fall took place rather rapidly, with little time for the development of intermediate phreatic levels. If so, this supports the case for valley incision and resurgence lowering as a result of glacial erosion during one of the sequence of glacial advances experienced by the Picos de Europa in the Quaternary.

The present local phreatic level in the cave, as indicated by the altitude of the terminal sump, is 1110 m, although the altitude of the inlet sump and the existence of active phreatic inlets in the walls of the streamway indicate that some variation around this figure exists. Consequently, contemporary flow along the streamway is vadose, with the result that in many places the streamway exhibits the classical keyhole-type form of a vadose-entrenched phreatic tube. This entrenchment has left the vadose tributary hanging at least 0.6 m above the height of the present thalweg, indicating that the tributary has been largely inactive, at least during the final episode of vadose incision along the streamway. Nevertheless, the sedimentological evidence within the main streamway shows that its development has been far more complex than the morphological evidence would suggest. In many places, particularly upstream of the T Junction, there is evidence of many phases of coarse fluvial-sediment input, and some overbank deposition, separated by phases of stalagmite deposition (Figure 2). The present cave floor at these points consists of an armoured bed of pebbles and cobbles. This is probably reworked from the surrounding sediment banks, since such deposits are absent where no fossil deposits occur.

Downstream of the T Junction, the main streamway passes through a number of chambers which appear to be genetically unrelated to the contemporary hydrological system. This is particularly so in the case of the Camp Chamber, where the present stream merely flows in and out of one corner of the chamber (Figure 1). These chambers and the somewhat higher sections of stalagmite-filled passage such as the Giga-stal Chamber appear to be older features which have been reinvaded by the present streamway.

Downstream of the Camp Chamber, the cave becomes more and more simply vadose, with few floor deposits and little evidence of past episodes of sedimentation in the form of stalagmite floors and fluvial deposits. Indeed, the only stalagmite appears to be that which is being presently deposited.

Finally, at the Cascades, the stream descends steeply by approximately 15 m over a series of waterfalls prior to reaching a long, gently-graded section of vadose streamway ending in the terminal sump. The Cascades may be a knickpoint resulting from vadose incision down to a lower phreatic level. On the other hand, the knickpoint could be the result of local structural control or an increase in total stream discharge where a major tributary joins the flow just upstream of the Cascades.

Cueva del Osu thus contains evidence of a relatively simple pattern of cave development, yet one which reflects major changes in the topography around the cave (Table 1). In particular, it can be seen that cave development has occurred in response to a sequence of progressively lower resurgence levels, themselves probably the result of episodes of valley incision. Although it is tempting to relate each phase of valley incision to that of episodes of glacial advance in the region, such a relationship cannot yet be justified on the available evidence and the establishment of a chronology must await the results of isotopic and magnetostratigraphic studies of speleothems from the cave which are now in progress.

References

Singleton, J. and Laverty, M., 1979. Cueva del Osu. Proc. Oxford Univ. Cave Club 9, 14-16.

Singleton, J. and Thwaites, D. , 1979. Stone-Lid Cave. Proc. Oxford Univ. Cave Club 9, 16

Figure 1: Cueva del Osu

 Table 1. Stages in the development of Cueva del Osu..