ALBERT

Notes: The architecture and sedimentary facies of a delta during its steady-state development varies with the type of distributary system, the basin water depth, and the different ways of sediment diffusion at the delta front. Analysis of these variables has resulted in twelve prototype deltas, which are postulated as sedimentological norms to facilitate comparisons of both modern and ancient delta systems.

Notes: Flume experiments show that current ripples on very fine sand surfaces always develop towards a linguoid shape with constant height and wavelength provided that sufficient time is allowed for their formation. Straight and sinuous current ripples only reflect intermediate stages in ripple development and may be regarded as non-equilibrium bedforms. The time period which current ripples require to reach linguoid equilibrium morphology is related to an inverse power of flow velocity. In the transitional stage from current ripples to upper stage plane bed (i.e. washed-out ripple stage) only the equilibrium wavelength remains constant, whereas equilibrium height rapidly decreases to zero.Our observations imply that bed-roughness parameters in sediment transport calculations can be simplified when equilibrium conditions are attained, and that inferences about flow energy from the dimensions of current ripples in very fine sand need to be regarded with caution.

Notes: Flume experiments were performed to study the flow properties and depositional characteristics of high-density turbidity currents that were depletive and quasi-steady to waning for periods of several tens of seconds. Such currents may serve as an analogue for rapidly expanding flows at the mouth of submarine channels. The turbidity currents carried up to 35 vol.% of fine-grained natural sand, very fine sand-sized glass beads or coarse silt-sized glass beads. Data analysis focused on: (1) depositional processes related to flow expansion; (2) geometry of sediment bodies generated by the depletive flows; (3) vertical and horizontal sequences of sedimentary structures within the sediment bodies; and (4) spatial trends in grain-size distribution within the deposits. The experimental turbidity currents formed distinct fan-shaped sediment bodies within a wide basin. Most fans consisted of a proximal channel-levee system connected in the downstream direction to a lobe. This basic geometry was independent of flow density, flow velocity, flow volume and sediment type, in spite of the fact that the turbidity currents of relatively high density were different from those of relatively low density in that they exhibited two-layer flow, with a low-density turbulent layer moving on top of a dense layer with visibly suppressed large-scale turbulence. Yet, the geometry of individual morphological elements appeared to relate closely to initial flow conditions and grain size of suspended sediment. Notably, the fans changed from circular to elongate, and lobe and levee thickness increased with increasing grain size and flow velocity. Erosion was confined to the proximal part of the leveed channel. Erosive capacity increased with increasing flow velocity, but appeared to be constant for turbidity currents of different grain size and similar density. Structureless sediment filled the channel during the waning stages of the turbidity currents laden with fine sand. The adjacent levee sands were laminated. The massive character of the channel fills is attributed to rapid settling of suspension load and associated suppression of tractional transport. Sediment bypassing prevailed in fan channels composed of very fine sand and coarse silt, because channel floors remained fully exposed until the end of the experiments. Lobe deposits, formed by the fine sand-laden, high-density turbidity currents, contained massive sand in the central part grading to plane parallel-laminated sand towards the fringes. The depletive flows produced a radial decrease in mean grain size in the lobe deposits of all fans. Vertical trends in grain size comprised inverse-to-normal grading in the levees and in the thickest part of the lobes, and normal grading in the channel and fringes of the fine sandy fans. The inverse grading is attributed to a process involving headward-directed transport of relatively fine-grained and low-concentrated fluid at the level of the velocity maximum of the turbidity current. The normal grading is inferred to denote the waning stage of turbidity-current transport.

Notes: Both a modern and a raised, late Holocene gravelly beach have been studied along the cliffed coast near the village of Chora Sfakion in southwestern Crete, Greece. The Holocene beach shows the record of regional, well-documented relative sea-level changes, in the form of regressive and transgressive sequences. The texture, fabric and depositional architecture of these sequences are described and compared with those of the modern, microtidal beach there. The depositional history of the raised beach is discussed in terms of the sediment transport processes and relative sea-level changes.The regressive sequences of both the modern and the Holocene beach resemble those of the mesotidal beaches in the Sker district of southwest Wales, described by Bluck (1967). The transgressive sequence of the Holocene beach resembles the mesotidal beach sequence near Newton (southwest Wales), recognized by Bluck as another type of progradational (regressive) beach. Therefore, transgressive beach sequences in the stratigraphic record might easily be overlooked or misinterpreted as regressive. The stratigraphic depositional patterns of gravelly beach systems are, apparently, still poorly understood.

Notes: Pebbly sediments of the shallow marine Abrioja fan-delta show pockets (bowl-shaped structures, partly filled with pebbles) and pillars (elongate structures, filled with sand and pebbles). These structures are most abundant in pebbly sediments deposited on a steep slope (ca. 25°-10°) and are absent in conglomerates deposited on a slope of ca. 6° and less, although they are present in the pelitic top of these beds.The pocket and pillar structures are interpreted as fluid escape structures originating from local liquefaction and fluidization, processes which are favoured by rapid deposition, rapid sediment accumulation, the presence of less permeable layers and an immature sediment texture.These conditions are met in conglomeratic fan-deltas, which have steep slopes with immature sediments. It is concluded that the presence of fluid escape structures in conglomeratic sediments may indicate a steep depositional slope.