Incised valleys on the Algarve inner shelf, northern Gulf of Cadiz margin: Stratigraphic architecture and controlling factors in a low fluvial supply setting

A network of cross-shelf paleovalleys has been recognized over the paleo-inner shelf off the Gilão-Almargem Estuary, a small fluvial drainage system that presently receives minor sediment supply in the eastern Algarve shelf, northern margin of the Gulf of Cadiz (SW Iberian Peninsula). This study is aimed at determining the driving controls that triggered substantially different paleohydrological conditions and sedimentary dynamics of ancient fluvial systems in this margin. We focus on evidences of secondary controls on valley genesis and evolution, superimposed to primary glacio-eustatic control such as antecedent geology, low fluvial supply and changing hydrodynamic regimes. The architecture and spatial distribution of these paleovalleys were interpreted based on a grid of seismic profiles with different resolutions. Likewise, a sediment core obtained in a distal position of the paleovalley system provided information about sedimentary processes during the most recent stage of valley infilling. The chronostratigraphic framework was constructed based on regional seismic horizons defined in previous studies and complemented with two AMS 14C dates obtained from bivalve shells. The inner shelf paleovalley system is composed of several incised valley features which exhibit a remarkable similar internal architecture. These inner valley features exhibit two major incision phases (from oldest to youngest; IP 2 and IP 1) that are thought to constitute a simple paleovalley system formed during the last glacial cycle. The origins of the incision are considered to be different. The older one is related to fluvial incision during the sea-level fall leading into the Last Glacial Maximum, whereas the recent one is interpreted as the result of tidal scour during the postglacial transgression. Their corresponding infillings are interpreted, respectively, as estuarine bay-fill deposits and estuary-mouth sands. Overlying the paleovalley infilling, a distinctive reflective unit is in agreement with the generation of coastal barriers and related depositional systems. The formation of the paleo-inner-shelf paleovalley system was strongly conditioned by antecedent geology, which strongly limited the generation of wide incised valleys and determined the amount of incision landward of a well-defined break of slope. Its postglacial infilling was mainly estuarine in nature, likely involving the development of a dendritic system, with numerous barriers interrupted by tidal inlets and channels in a mixed estuarine system with low fluvial supply

Mangarara Formation: exhumed remnants of a middle Miocene, temperate carbonate, submarine channel-fan system on the eastern margin of Taranaki Basin, New Zealand

The middle Miocene Mangarara Formation is a thin (1–60 m), laterally discontinuous unit of moderately to highly calcareous (40–90%) facies of sandy to pure limestone, bioclastic sandstone, and conglomerate that crops out in a few valleys in North Taranaki across the transition from King Country Basin into offshore Taranaki Basin. The unit occurs within hemipelagic (slope) mudstone of Manganui Formation, is stratigraphically associated with redeposited sandstone of Moki Formation, and is overlain by redeposited volcaniclastic sandstone of Mohakatino Formation. The calcareous facies of the Mangarara Formation are interpreted to be mainly mass-emplaced deposits having channelised and sheet-like geometries, sedimentary structures supportive of redeposition, mixed environment fossil associations, and stratigraphic enclosure within bathyal mudrocks and flysch. The carbonate component of the deposits consists mainly of bivalves, larger benthic foraminifers (especially Amphistegina), coralline red algae including rhodoliths (Lithothamnion and Mesophyllum), and bryozoans, a warm-temperate, shallow marine skeletal association. While sediment derivation was partly from an eastern contemporary shelf, the bulk of the skeletal carbonate is inferred to have been sourced from shoal carbonate factories around and upon isolated basement highs (Patea-Tongaporutu High) to the south. The Mangarara sediments were redeposited within slope gullies and broad open submarine channels and lobes in the vicinity of the channel-lobe transition zone of a submarine fan system. Different phases of sediment transport and deposition (lateral-accretion and aggradation stages) are identified in the channel infilling. Dual fan systems likely co-existed, one dominating and predominantly siliciclastic in nature (Moki Formation), and the other infrequent and involving the temperate calcareous deposits of Mangarara Formation. The Mangarara Formation is an outcrop analogue for middle Miocene-age carbonate slope-fan deposits elsewhere in subsurface Taranaki Basin, New Zealand

Global distribution of organic carbon content and age in submarine canyons: results from the Factors influencing the Accumulation of organic Carbon and its Transfer in Submarine canyons (FACTS) project

6th International Submarine Canyon Symposium (INCISE-2023), 4th to 8th December 2023, Wellington, New ZealandSubmarine canyons are ubiquitous geomorphological features of continental margins that serve as important pathways of organic carbon (OC) transfer from coastal environments to deep-sea basins. Through the FACTS project, we compiled surficial sedimentological (e.g., grain size, mineral surface area) and geochemical (e.g., OC, ẟ13C, Δ14C) data from 48 submarine canyons incising continental margins worldwide to understand the processes that modulate the variations in the quantity, origin, and age of OC. Despite a general fining of surficial sediments with increasing depth, OC content generally decreased downcanyon in all submarine canyons. This trend is attributed to the degradation of OC with increasing transit time, as observed by the general decrease in OC loading. The downcanyon degradation of OC is further supported by the general decrease of Δ14C, indicating an overall ageing of OC with across-margin distance. However, certain submarine canyons (e.g. Calahonda and Mackenzie canyons) did not follow this trend, and instead, presented a general increase in Δ14C, due to the deposition of older (e.g., petrogenic) OC in the canyon head that became progressively diluted by fresh marine OC input offshore. These results reveal that the primary driver of the contrasting OC signatures accumulating in submarine canyons worldwide is the origin and age of the OC delivered into the continental margin, whereas post-depositional processes promote its downcanyon ageing. To better understand the factors that affect the distribution of the origin of OC in submarine canyons, we analysed biomarkers (proteins, lipids and specific fatty acids, carbohydrates, as well as phytopigments) and compound-specific-isotope-analyses (CSIA) in fatty acids of three contrasting submarine canyons incising the Gulf of Palermo, Sicily. Although all canyons presented a decreasing contribution of terrestrially-derived biomarkers downcanyon, Oreto Canyon did not present the greatest terrestrial fraction despite its proximity to a river mouth. Instead, Eleuterio Canyon, located downcurrent within the Gulf, had the greatest terrestrial OM contribution. These results reveal that along-margin hydrodynamic processes also govern the dispersal of OM across these geomorphological features

Submarine landslides on the Great Barrier Reef shelf edge and upper slope: a mechanism for generating tsunamis on the north-east Australian coast?

Shallow (< 200 m) submarine landslides influence margin evolution and can produce devastating tsunamis, yet little is known about these processes on mixed siliciclastic-carbonate margins.\ud \ud We have discovered seven landslides along the shelf edge and upper slope of the central Great Barrier Reef (GBR), Australia. The largest shelf edge landslide is investigated in detail and represents a collapse of a 7 km long section of the shelf edge at 90 m water depth with coarse debris deposited up to 5.5 km away on the upper slope down to 250 m.\ud \ud The precise timing and triggering mechanisms are uncertain but available chronologic and seismic stratigraphic evidence suggest this event occurred during the last deglacial sea-level rise between 20 and 14 ka.\ud \ud Regional bathymetric data confirms that these shelf edge and upper slope slides are restricted to the central GBR between latitude 18° and 19°S, suggesting a spatial relationship between the extensive Burdekin paleo-fluvial/delta system and shallow landslide activity.\ud \ud This study highlights an important local mechanism for the generation of tsunamis on this margin type, and numerical simulations under present conditions confirm a 2 to 3 m tsunami wave could be produced locally.\ud \ud However, we consider that the risk of such slides, and their resulting tsunamis, to the modern coastline is negligible due to their relatively small size and the capacity of the GBR to dissipate the wave energy