Ichnology of a true substrate in a Cretaceous ephemeral pond, Neuquén Basin, Argentina

PAZOS, PABLO J.; HEREDIA, ARTURO M.; COMERIO, MARCOS A.; FERNÁNDEZ, DIANA E.; GUTIÉRREZ, CAROLINA

Florianópolis

Congreso; 5th International Congress on Ichnology; 2024

5th ICHNIA organizing committee, International Ichnological Association

In the last decade, there has been a growing interest in the study of bedding planes, particularly those candidates to be true substrates, capturing the attention of sedimentologists. A true substrate records the sediment-water/air interphase at the time of deposition [¹], resulting in an archive of processes that occurred during a very brief taphonomic window, where stasis prevailed [1,2]. During this period, microbial activity occurs and provides substrate resistance preventing later erosion. In this scenario, trace fossils assist in the identification of true substrates. These trace fossils must be disregarded as undertraces, and other sedimentary structures like certain ripple types and desiccation cracks are also documented in the candidate surface. Other accompanying indicators, such as microbially induced sedimentary structures (MISS) [3], are usually present. In the Cretaceous of the Neuquén Basin, on a biostabilized surface in the Agrio Formation, trace fossils with delicate features exquisitely preserved, along with water level marks and steeply inclined lee-side ripples, supported a true substrate interpretation [4]. Later studies on the same surface, in physical continuity with the previously studied channel, documented current ripples preserved on a gently inclined plane (channel border), that extends almost horizontally and contains tracks of slipping dinosaurs on a biostabilized surface [5]. This true substrate allowed the reconstruction of a marginal-marine scene documented in a short taphonomic window. In the nonmarine Candeleros Formation, a narrow-gauge sauropod trackway with rims presents almost undisturbed ripples on top due to biostabilization suggesting a true substrate [6]. We study here other surface documented in the same nonmarine stratigraphic unit where the previous works [6] suggested arid conditions based on halite pseudomorphs, adhesion and wind ripples, together with widespread mud cracks in iron rich cemented fine-grained sandstones and mudstones. The herein studied surface covers an area of approximately 120 m2, but the exact limits are unknown due to covering by overlying beds. The area represents a very shallow unfilled exposed depression, specifically a shallow ephemeral pond. Ephemeral ponds, usually not considered in the study of trace fossils assemblages or icnofacies analysis, are extremely shallow bodies of waters often found in flat areas, frequently linked to fluvial plains, and their permanence depends on climate conditions. Within the pond, sedimentary structures and bioturbation vary in location and type. Ripples and desiccation cracks are dominant, and vertebrate tracks prevail over invertebrate trace fossils. The combined study of sedimentary structures and trace fossils indicates different stages in the pond?s evolution until complete dryness. The earliest stage documents a shallow water level, where wave ripples some with flat-topped crests (Fig. 1A), and others with abrupt changes in axis suggest some emerged areas (Fig.1B). This stage implies recurrent biostabilization, documented by pockets with multidirectional ripples (Fig. 1A). In some cases, invertebrate trace fossils are partially covered by ripples or are found in flat areas intensely affected by desiccation cracks. The invertebrate trace fossils documented in patches with ripples consist of curved or radiate probing projections departing from a center, reaching 5 cm in length along their longest axis. They are assigned to Oldhamia Forbes with the ichnospecies O. curvata Lindholm and Casey and O. radiata Forbes. The latter is more abundant. This new finding constitutes the first record of the ichnogenus in nonmarine deposits in the world. Similarly to Cambrian marine records of Ireland, Arenicolites Salter is also documented [7], and the possibility of the same producer cannot be disregarded. The final stage involves the complete dryness of the pond, marked by the presence of numerous desiccation cracks. In some areas, classical mud cracks with sandstone infilling were observed (Fig. 1C). However, the most frequently observed cracks exhibit a pattern consistent with the desiccation of biostabilized surfaces. They include irregular, tortuous, incipient, and reticulated cracks, exposed horizontally or irregularly turned upward (Fig. 2A), forming areas of uneven topography with local variations in the cracking pattern related to biostabilized surfaces. The intermediate stage marks a period of stasis with progressive changes in substrate moisture, which is only recognizable solely through the record of imparted tracks on the surface. A sauropod-inferred trackway and isolated tracks were produced prior to cracking (Fig. 2B), and the observed rims are irregular, suggesting a high level of moisture. Some isolated tracks without rims but deeply imprinted probably record a more advanced stage of dryness. It is essential to note that the entire area with many sauropod tracks is scarcely cracked. Theropod tracks imparted on the surface are crucial to understand the progressive dryness of biostabilized areas, documenting the shortest taphonomic window in the entire evolution of the substrate. They include different morphological variants and sizes. Firstly, in a small area without cracks, an isolated grallatorid-type track quite deeply impressed on the surface presents sharp borders (Fig. 2C), suggesting a still wet substrate. The question arises: why just one track? Close to the track, most of the surface is cracked, indicating that the dinosaur mostly walked on a surface with less moisture and in a more advanced state of dryness and only in a wet patch the track was well recorded. A second type of theropod track consists of larger ones forming a challenging-to-follow trackway, presenting wider and larger digit impressions than the first type. Finally, an extremely flat and the largest tridactylar track (Fig. 2D), isolated, is almost topographically indistinguishable from the surrounding area. A possible explanation for the noted differences in the track preservation is related to progressive dryness but variations in the speed and size of the producer might have resulted in different pressure on the substrate. Interestingly, very shallow oval hollows are observed in almost all areas of the exposed surface. They are interpreted as transmitted sauropod tracks (undertracks), spectacularly preserved in vertical section in overlying beds, with some of them in contact with the studied surface. The studied surface is an excellent example of a true substrate but also a clear palimpsest in the sense of Davies and Shillito [2], comprising a series of abiotic and biotic processes that show the complexity of a ?blink? in the geological record. This work also emphasizes the need of sedimentological analysis focused on the substrate during the study of vertebrate tracks and trackways. In conclusion, this work documents that true substrates may result in excellent archives of their moisture variations. On those surfaces, vertebrate tracks provide crucial information not only about intermediate stages in pond evolution but also regarding the ichnofauna diversity that is higher than in any other part of the stratigraphic unit.