LOOKING FOR CRITERIA TO DISCRIMINATE KEY STRATIGRAPHIC SURFACES IN MONOTONOUS SAND-RICH TIDE-MODULATED SHOREFACE SUCCESSIONS

OLIVO, MARIANA S.; ISLA, MANUEL F.; MOYANO-PAZ, DAMIÁN; HALPERN, KAREN; MUÑOZ, DIEGO F.; DE LA PUENTE, G. SUSANA; ARREGUI, MARIANO

La Plata

Congreso; XVIII Reunión Argentina de Sedimentología : IX Congreso Latinoamericano de Sedimentología; 2023

Asociación Argentina de Sedimentología

Sequence-stratigraphic surfaces in shallow-marine successions are evidenced by vertical changes in the facies stacking pattern related to shoreline shifts (regression and transgression). Then, identifying vertical changes of facies variability requires determining deepening or shallowing relationships, mostly interpreted from abrupt textural changes. This universal criterion can be difficult to apply in some monotonous successions with low facies variability. For example, the record of macrotidal shorefaces is characterised by uniform calibres between distal and proximal facies, the low potential of mud preservation in the lower shoreface, and the intense bioturbation of sand-rich deposits. Hence, more subtle alternative approaches are needed to recognise the sequence-stratigraphic surfaces within this succession type.The lower Palaeozoic Balcarce Formation, interpreted as a tide-modulated shoreface-shelf succession, includes offshore transition (OT), lower shoreface (LS), upper shoreface (US) and foreshore (FS) facies, and makes an outstanding case for sequence-stratigraphic analysis. We analyse the Balcarce Formation along WNW-ESE and NE-SW-trending transects near the Balcarce locality. This unit comprises several coarsening- and shallowing-upward successions separated by four orders of bounding surfaces (S1-S4). S1 is erosional and extends more than 20 km in down-dip trends. It shows gravel-sandy US/delta front facies with abundant Diplocraterion and Skolithos overlying non-bioturbated sandy FS or US deposits, suggesting a regressive surface for S1. S2 is up to 20 km long and is represented by up to a metre-thick OT or LS deposits overlying US units or FS facies. S2 is highlighted by the occurrence of Cochlichnus, Herradurichnus, and Thalassinoides? when caps US facies. It is interpreted as a transgressive surface. S3 is similar in length to S2 but shows different expressions from distal to proximal sectors. In distal areas, OT deposits less than 0.1 m-thick overlie US units. S3 exhibits abundant Cruziana, Diplichnites, Monomorphichnus, Psammichnites and Rusophycus. In proximal areas, S3 can be either defined by an erosional contact recorded between non-bioturbated (above) and coarse intensely bioturbated US deposits (below) or by US facies overlying FS sediments. S3 represents a transgressive surface with a minor deepening degree than S2. S4 is only identified in distal areas, where it is less than 20 km long. S4 marks coarser US units overlain by finer-grained US facies; it lacks sequential meaning but it might suggest high-frequency bedform migration and abandonment cycles, during which fines were deposited within troughs between units.This analysis shows how the tide- and wave-related facies organise from distal to proximal settings of a tide-modulated shoreface. Different deepening degrees supported by vertical facies shifts and ichnogenera associations allowed us to distinguish major (S1) to minor (S4) stratigraphic surfaces representative of different magnitudes of change. S1 to S3 indicate shoreline shifts and suggest major to minor orders of relative sea-level changes, respectively. S4 does not show shoreline displacements, but intrinsic system responses to tidal and wave regimes. This contribution helps to improve existing sequence-stratigraphic models for tide-modulated shoreface successions, and delves into the understanding of the Balcarce Formation about the long-short-term evolution of this type of environment.