Variations of substrate-controlled ichnofacies along a marine sequence boundary: an example from the Valanginian of central Neuquén Basin (Argentina)

Abstract

Erosion and/or non-deposition associated with basin-scale stratigraphic discontinuities such as regressive surfaces of marine erosion (RSME) and sequence boundaries (SB) can result in the formation of different substrates including non-cohesive sands, dewatered muds and/or cemented hardgrounds. Thus, any individual discontinuity across a basin could be demarcated by different substrate-controlled trace fossils, but ancient examples supporting this concept have not been widely reported. This contribution discusses a case study from the Intra-Valanginian Unconformity, a discontinuity that represents a dramatic sea-level fall within the Lower Cretaceous strata of the Neuquén Basin (Argentina). This second-order sequence boundary can be mapped for 10’s of kilometers both along depositional strike and dip. In the proximal region of the lowstand configuration, the subaerial segment of the SB is demarcated by alluvial conglomerates (basal Mulichinco Fm) onto deep-marine black shales (Vaca Muerta Fm). In the marine realm, the time-equivalent RSME separates black shales and marls beneath from shallow-marine carbonate deposits above. The downdip expression of the RSME is well-exposed across a 20-km long, strike-oriented outcrop. Everywhere in this region the discontinuity is demarcated by robust Thalassinoides that penetrate up to 25 cm in the underlying succession. These burrows have well-defined walls and spherical to subspherical cross-sections, suggesting that the substrate was relatively firm during excavation. The burrow system is filled mostly with bioclasts of echinoids and bivalves from overlying sediments, but also with pebble-size mudstone intraclasts that were eroded from the underlying succession. This burrow system is interpreted to represent the Glossifungites Ichnofacies. The updip expression of the RSME was recorded in subsurface from a cored hydrocarbon well located about 20 km from the outcrops. As in the distal region, a first generation of passively filled Thalassinoides excavated into a firm substrate is attributed to the Glossifungites Ichnofacies. However, this suite is in turn cross-cut at the top by mm-long borings representing the Trypanites Ichnofacies. The RSME records a complex erosional history after the onset of the Valanginian sea-level fall. The more proximal regions of the marine realm underwent erosion and exhumation of buried sediments in the first place, allowing for the formation of crustacean-colonized firm substrates. As sea level continued falling, these processes extended (or migrated) to distal areas. Meanwhile, long-lived by-pass and/or early marine cementation favored the development of hard grounds in the proximal region (subaerial exposure, although possible, cannot be unambiguously established), producing the co-occurrence of different substrates at this time. Shallow-marine sediments probably deposited during early transgression and backstepped onto the RSME, producing a composite RSME/Transgressive Surface.