Controls on strath terrace formation and evolution: The lower Guadiana River, Pulo do Lobo, Portugal

Abstract

The lower Guadiana River carves a wide canyon in the Variscan metamorphic rocks of the SW Iberian Peninsula and presents an exceptionally homogeneous and well-preserved case of a strath terrace (ST) some 40 km long and 400 m wide. The rocky surface or strath shows different characteristics and development stages. Upstream, the lateral erosion level forms a flooded riverbed, or submerged strath, that gradually rises above the thalweg and develops into an active floodplain. Downstream, below the 15 m high Pulo do Lobo waterfall, the strath is incised by an inner canyon that forms a terrace on both sides. The inner channel increasingly widens and the ST loses its longitudinal continuity, yet remains prone to flooding during extraordinary magnitude floods. To assess the factors controlling the formation of this ST, we examined the roles of key drivers of ST formation: lithological hardness, tectonic structures and climate change, using cosmogenic data to create a chronologic context. The ST developed favoured by a monotonous succession of phyllites, as shown by Schmidt hammer resistance data, and interbedded more resistant quartz veins do not significantly alter its flat configuration. Variscan structures and measured joints determine the main orientation of the river and control the drainage network pattern. The intersection of the NS and NE-SW fractures promotes a wavy surface due to three-dimensional minor fold trains that act as discontinuities and play an important role in ST erosion through quarrying and plucking. However, we detected no evidence of ST deformation attributable to recent or Late Cenozoic tectonic activity.

The upstream active riverbed progressively changes downstream into an ST. This implies that Be radiometric ages obtained at Pulo do Lobo may not represent the ST's genetic age but rather the time of dismantling of its alluvial or sedimentary cover. This means the strath surface formed prior to its cosmogenic age and was therefore controlled by a higher base level, indicating a higher sea level than the current one, given the downstream area studied. The Quaternary sea level fluctuations show records of only a few meters above the current level. Instead, the sea level low stand maintains an average depth of −60 m, reaching −140 m during the last glacial period. This low base level context could justify the strong incision of the inner channel within the ST, but not the age of strath surface genesis and expansion and thus this surface should have evolved previously. The highest possible sea level stand corresponds to the Pliocene transgression, which reached an elevation of 120 m in the region, covering Miocene paleorelief. The mouth of the Guadiana River at that time resembled a fluvio-deltaic embayment between the Algarve domain and the Guadalquivir depression, which was infilled with fluvial deposits at the end of the transgression. Collectively, these data suggest that cosmogenic dating would define the time of exhumation of this Pliocene sedimentary cover.

Despite great variation of the ST ages, estimated incision rates at the knickpoint area are consistent with the total entrenchment of the Pulo do Lobo fall, especially if we consider that data correspond to an active erosion site. Highest erosion rates were found for the pothole area compared to those obtained for the higher ST surface. Radiometric ages correspond to the period of lowest Quaternary sea level and during which headward erosion would have been very active in the inner channel. The waterfall area shows more resistant lithology in its concentrated quartz veins, but this effect is inconspicuous at the ST surface. The regular gradient of 0.02 m/m and vast extension of this perched strath, linked upstream to the Guadiana riverbed, indicates a relic exhumed surface beneath its sedimentary cover. Notwithstanding the subsiding activity of the Gulf of Cádiz, the continental margin behaves as a tectonically stable block, which allows for the persistence of former morphologies in these ancient landscapes reshaped by bedrock river dissection.