Most of the described material was studied in situ during three field seasons in September 2002 and February and May 2003 and from field photographs. The deposits of the Patagonia Formation at Las Grutas are quite friable and the collection of large, relatively complete burrow systems was not possible. More resistant, elementary components were collected and rapidly hardened in the field with glue, and parts of large structures were cast in the field using silicone. In the lab positive molds of the silicone casts were made using plaster of Paris.
The diameter of the inner tube was carefully measured from amplified digital photographs in a PC screen. Part of the terminology concerning the geometry of the backfilling, such as the terms, spiral, helicoidal or conical, used in the systematic ichnology section, is fully explained later in a special section.
The material and molds are housed at the Centro Austral de Investigaciones Científicas (CADIC) under the numbers CADIC PI 21 to 37.
Geologic setting
The general stratigraphy at Las Grutas consists of limited exposures of Paleozoic granitoids at Piedras Coloradas, which are unconformably covered by more extended, shallow marine and tidal, tuffaceous mudstones, siltstones, and fine sandstones of the Patagonia Formation (Miocene). Just south of Las Grutas, a distinctive Pleistocene fossiliferous, hardcemented conglomerate is exposed in the intertidal zone (figure 1.A; Angulo et al ., 1979).
Our study is concentrated on the exposures of the Patagonia Formation along the sea cliff between Las Grutas and La Rinconada (figure 1.A). Here the Patagonia Formation consists of a fully bioturbated succession, ca . 12 m thick (figures 1.B, 2), which is very gently tilted (about 1°) to the NE. The oldest sediments are exposed to the SW, largely in the intertidal zone of the main beach area at Las Grutas, and consist of yellowish, densely bioturbated and mottled mudstones, with minor intercalated thin sandstone beds. Poorly preserved marine gastropods and bivalves are relatively common. They are overlain in sharp unconformity by reddish, bioturbated, heterolithic mudstones and sandstones with frequent ripple crosslamination, flaser bedding, mud drapes, and herringbone cross-stratification, informally referred here to units I and II. These are differentiated by the relative proportion and thickness of mudstones and sandstones; unit I is dominated by mudstones with alternations of thin sandstone beds and unit II is dominated by sandstones with thin mudstone beds (figures 1.B, 2). Some of the sandstone beds of unit II bear a dense concentration of fecal pellets.
Part of the bedding in the reddish, heterolithic succession of units I and II is at an angle with respect to the lower and upper strata and represents inclined heterolithic stratification. Unit II is sharply cut in angular relationship by horizontal, whitish beds of unit III, and this angular contact is best seen at the sea cliff just SW of La Rinconada. Unit III consists of a basal, lenticular conglomerate, and sandstone and siltstone beds of contrasting white color (figures 1.B, 2). They are highly bioturbated and record relatively abundant but poorly preserved serpulid tubes, mollusks, bryozoans, and crabs. The unconformities at the base of unit I and unit III are associated with typical components of the Glossifungites ichnofacies (figures 1.B, 2, cf . Frey and Seilacher, 1980), including galleries of Thalassinoides isp.; U-tubes of Diplocraterion isp.; Gastrochaenolites isp.; and distinctive burrows that cut across a dense, mottled bioturbated structure with sharp, neat boundaries indicating that they were excavated in a firm, semiconsolidated substrate.
On the basis of the features described above, the heterolithic beds of units I and II are interpreted as tidal flats and migrating intertidal channel deposits. The unconformity at the top of unit II probably represents a ravinement surface of erosion, covered by shallow marine, fossiliferous sandy siltstones.
Cylindrically laminated burrows are particularly abundant in the reddish, heterolithic deposits. Several, distinct burrow systems are recognized in this paper, and their occurrence is closely tied to a particular grain size of the host sediment (figure 1.B). The mud-dominated unit I bears abundant decapod crustacean trace fossils, Asterosoma radiciforme von Otto, Patagonichnus calyciformis n. isp., and rare Asteriacites isp. The sandier unit II is dominated by Asterosoma radiciforme and Patagonichnus stratiformis n. isp., with some Patagonichnus thalassiformis n. isp. at its top. Large, vertical tubes of Ophiomorpha nodosa , most of them passively filled with concentric laminae, are present in both units I and II. In addition to a characteristic dense, mottled structure, unit III records two levels with dense concentration of Patagonichnus thalassiformis n. isp. in sandy siltstones and tuffs (figure 1.B).
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