Graham Evans

Rias, estuaries and incised valleys: Is a ria an estuary?

Abstract Today, the term ria is rarely used by sedimentologists or stratigraphers, although it embodies all the characteristics of the so-called incised valleys – a topic of considerable interest since the increased understanding of the three dimensions of transgressive sequences using seismic stratigraphy. When it is used, it is usually referred to as a type of estuary; however, in many cases, only a minor part is influenced by estuarine processes. It is a useful term which should be revived as it encompasses in its entirety all the features and resulting deposits of an incised valley whether it is estuarine or marine.

Metals in the sediments of Ensenada de San Simon (inner Ria de Vigo), Galicia, NW Spain

Abstract The Ensenada de San Simon is the inner part of the Ria de Vigo, one of the major mesotidal rias of the Galician coast, NW Spain. The geochemistry of its surface sediments, and the river sediments which drain into it from a granitic and metamorphic hinterland, are described. Multivariate statistical analysis of the sediment compositions (using ridge regression and mixture-modelling) enabled the major- and trace-element distributions to be accounted for in terms of both natural and anthropogenic sources: Between 60 and 80% of the Cr, Cu, Ni, Pb and Zn concentrations of the bottom sediments of the Ensenada can be explained by sediment input from the combined R. Oitaben and R. Verdugo, the R. Redondela and faecal matter from mussel rafts, but there is additional enrichment towards the mouth of the Ensenada. This enrichment is attributed to marine water entering the Ensenada from the polluted industrial areas of the adjacent Ria de Vigo. It is suggested that these metals are carried landwards in solution by the tidal incursion of marine water (the volume of which, on an annual basis, greatly exceeds that contributed by the rivers). Although the incoming marine waters may also be important in supplying Pb to the outer Ensenada, this element may also be delivered by land run-off, or by windborne vehicular emissions to the Ensenada as a result of the surrounding network of roads and a road bridge over the Estrecho de Rande.

Upper slope sediment waves in the Cilician Basin, northeastern Mediterranean

Large sediment waves have been observed within the upper slope deposits of the Cilician Basin (northeastern Mediterranean), at the interfluve between two submarine canyons present offshore of the Mersin Shelf. There are several generations of sediment waves stacked within the sedimentary sequence, with the most recent bedforms found on the seabed in an area consisting of fine-grained sediments. The surficial sediment wave field, estimated to cover an area of ~55 km2, is found at water depths between 250 and 310 m. The buried sediment wave fields have similar dimensions, but they are located further downslope. Wave dimensions increase with water depth and depth in the sedimentary sequence. The largest bedforms reach 40 m in height and 1.8 km in length. Most waves appear to have been migrating upslope, i.e. towards the north/northeast, and this migration direction is mostly consistent throughout the sedimentary sequence. This consistency indicates similar mechanisms of formation and maintenance over a considerable time interval. The morphology and migration pattern of the observed bedforms suggests that sedimentation in the Cilician Basin during wave formation has been controlled by near-bed flows resembling those generated by the present Asia Minor Current, although these flows may have been stronger in the past than they are at present

The flux of siliciclastic sediment from the Iberian Peninsula, with particular reference to the Ebro

Abstract Today, the main flux of sediment from the Iberian Peninsula is carried to the Atlantic Ocean by westerly or northwesterly flowing rivers. The only major contribution of sediment from Iberia to the Mediterranean is that carried by the Ebro River, which flows along a foreland basin before cutting through the Catalan Ranges to form a delta in the adjacent Valencian Trough of the Mediterranean. During the Cretaceous and Early Eocene the sediment flux was to the northwest in this part of Iberia. During Late Eocene-Tortonian times it was mainly centripetal, as in other Tertiary Basins in Iberia. Beginning in the Seravallian-Tortonian the main flux of sediment has been to the adjacent Valencian Trough of the Mediterranean. This change in direction, it is proposed, was initiated in Pre-Messinian times by a small river cutting back into the Ebro Basin by headward erosion and was possibly aided, or even induced, by changes in palaeo-slope during contemporaneous rifting along the Mediterranean coast.

Sediment fluxes and the evolution of a riverine-supplied tectonically-active coastal system: Kyparissiakos Gulf, Ionian Sea (eastern Mediterranean)

Abstract Kyparissiakos Gulf, located in the southwestern part of Greece (northeastern Ionian Sea), is a riverine-coastal system that has developed over the southern flank of the Alpine orogenic belt (Hellenides). Some 4 km in the vertical separates the heights of the mountain peaks to the depths of the adjacent offshore deep-water basin. This system extends horizontally over approximately 100 km. The area experiences intensive tectonism (e.g. seismicity), a Mediterranean type of climate and microtidal and moderate wave-energy oceanographic settings. Large quantities of sediments (>2.5 × 106t year−1), transferred principally by the River Alfios, are the product of denudation of the high relief (in excess of 2000 m), developed on erodible lithology (with siliciclastics and carbonates >90%) under moderate climatological conditions. The large amounts of sediments produced in the hinterland, in association with land-ocean process interaction, have led to the formation of a coastal zone that includes deltaic plains and coastal barriers with dune fields, which enclose lagoons. The shape and morphological characteristics of the shore zone indicate, clearly: (a) the dominance of the wave activity; (b) an overall northward longshore sediment movement; and (c) a major depocentre at the northern, naturally sheltered, end of the Gulf. Seawards, the coastal zone includes a narrow continental shelf covered with a blanket of recent sediments, which are terrigenous in origin; these extend down a steep slope, where materials is transferred to the deep (approximately 1800 m) offshore basin (the northward component of the Hellenic Trench) primarily by gravitational mass movements; these are triggered often by earthquake activity. To a first approximation, some 50% of the riverine sediment fluxes accumulate over the shelf, whilst another 25% is transported over the slope to the deeper ocean waters. The construction of (two) dams has led to a dramatic reduction in the sediment supply to the coast, having already caused the retreat of the River Alfios deltaic coastline, by >100 m. Similarly, artificial drainage of the lagoons (for the development of agricultural land) has affected the overall ecosystem (altering the the fauna and flora) of the coastal zone Thus, the Upper Quaternary (mostly Holocene) evolution of this particular coastal system is attributed, primarily, to processes and balances between sediment fluxes (e.g. terrestrial transportation, seaward dispersion) and, recently, to human interference.

Recent surficial shelf sediments of the Cilician Basin (Turkey), northeastern Mediterranean

Abstract The continental shelf of Mersin Bay forms the northwestern margin of the Adana-Cilician Basin; it extends from the Goksu delta (in the southwest) to the Seyhan-Tarsus-Ceyhan delta (in the northeast) and is narrower and steeper in the southwest than in the northeast. Five depositional zones are recognised on the basis of the analysis of surficial sediments: (1) a “ Modern Terrigenous Prism ”; extending from the coast to 50 m water depth; (2) a “ Shelly Zone ” on the mid-shelf (50–100 m), characterised by an abundance of skeletal debris; (3 and 4) “ Prodeltaic Zones (I and II) ” in the northeastern and southwestern parts of the shelf, where silty sediment from the major Seyhan-Tarsus and Goksu rivers, respectively, has been deposited; and (5) the “ Outer Shelf Zone ”, extending from the 100-m contour to the shelf edge, characterised by clay-sized sediment and abundant remains of planktonic micro-organisms. Sediment appears to be reaching the shelf from two main sources: (i) a lateral supply from the coastal rivers between the two major deltas, which is being dispersed along the coast to form the “ Modern Terrigenous Prism ” and is dominated by near-shore wave activity and wave-generated currents; and (ii) fine-grained sediment from the Seyhan-Tarsus rivers, transported along the shelf under the action of the cyclonic Mediterranean circulation. Some of the latter sediments settle to form the distal tongue of the Seyhan-Tarsus delta (Prodeltaic Zone I), whilst the remainder settles over part of the shelf to the southwest. The sediment supplied by the Goksu river is generally swept away to the southwest, by the southwesterly-flowing cyclonic Mediterranean Current and it influences the shelf only within its immediate vicinity to form Prodeltaic Zone II. The mid-shelf area (Shelly Zone), lying at water depths of between 50 and 100 m, receives some fine silt and clay. However, it is cloaked in a thin layer of skeletal debris, which is difficult to explain but may be partly of anthropogenic origin although this requires further investigation.

Some comments upon: Palaeozoic oolitic ironstones on the North American Craton (Van Houten, 1990)

Article original : Van Houten, F. B; 1990. Palaeogeogr; Palaeclimatol; Palaeoecol; 80 (3/4): 245-254.