George E. Farrow

Structures and processes in bayhead deltas: Knight and bute inlet, British Columbia

Abstract Two bayhead deltas that possess similar tidal, wave and discharge characteristics are described, and their sedimentary structures, sediment character and lithofacies are compared. Although existing models predict that the two deltas should possess similar sedimentary styles of progradation, they do not. Relative to the Homathko, the Klinaklini River has a slightly larger drainage basin, steeper thalweg, and markedly greater bedload movement onto the delta. The Klinaklini has a braided deltaic morphology that develops into 30 m deep submarine channels in its prodelta environment. Except for channel gravels, the exposed deltaic facies are composed of fluvially dominated linguoid ripples of sand intermixed with climbing ripples, ladder ripples, and tidally influenced lenticular and flaser bedding. The Homathko delta has only one channel on its western flank surrounded by levee, flood plain and mouth-bar deposits dominated by the river. The eastern flank consists of wave-built sand bars transgressing landward over a tidal-influenced mudflat. The Homathko prodelta environment is extremely rugged, with steep slopes and slump scars resulting in one mega-submarine channel 140 m deep. The combination of low salinity estuarine waters and high sedimentation rates (> 30 cm yr −1 ) results in an impoverished macrofauna in both deltas: physical structures can therefore be seen in environments where they are normally obliterated.

Bioclastic carbonate sedimentation on a high-latitude, tide-dominated shelf; Northeast Orkney Islands, Scotland

ABSTRACT Shell-sands and gravels cover much of the shallow Orkneys shelf at 59°N, accumulating locally into 30-m-high banks at rates up to 540 g/m2/yr (67 cm/l,000 yr). Overall, the Orkney shelf sedimentation rate is approximately 10 cm/l,000 yr, compared with 3 cm/1,000 yr for the entire Scottish continental shelf. Major sandbanks are located off headlands that produce circulation loops in the tidal flow. Regional sandwave orientations reveal a clockwise transport of sediment around the islands, probably resulting from storm-wave reinforcement of the tidal asymmetry combined with the net inflow of Atlantic water into the North Sea. Carbonate production is high from the Modiolus epifauna. Modiolus shell gravels, commonly containing Glycymeris, pass laterally into comminuted shell-sands. Within the euphotic zone (down to 40 m) dead shells are weakened by echinoid biting, algal boring, and limpet grazing. Boring by fungi and clionid sponges and grazing by chitons are common but not depth-restricted. Sediments contain 89-95% carbonate on the level bottom offshore, but 94-99% in sandwaves. Mean values for the main skeletal components are--bivalves 46%, barnacles 18%, bryozoans 11%, and serpulids 7%. Calcareous algal gravels occur in sheltered areas less than 20 m deep. Bryozoa typify lower-energy offshore environments, while more durable barnacle and serpulid debris is concentrated in sandwave fields. The sediments are dominantly calcitic and have a high preservation potential.

Molluscan grazing of sublittoral algal-bored shells and the production of carbonate mud in the Firth of Clyde, Scotland

The limpet Acmaea virginea is an important agent of erosion of bivalve shells within the photic zone. This lower limit is governed by the depth range of shell-boring algae on which it feeds, the dominant alga being conchocelis. Any authigenic ferromanganese coatings on shells are removed by the grazing and the shells may become wafer thin and highly fragmented. The chiton Lepidopleurus also grazes shell surfaces but feeds more on superficial detritus than on the boring algae, and so its depth range is not so restricted. Both molluscs leave highly distinctive radula marks on shells which are potentially preservable in the fossil record. Acmaea virginea produces broad scoops, six-pronged, with very sharp narrow interstitial ridges. Lepidopleurus produces short, sharp scratches in sets of two, three or four, with wide interstitial ungrazed plateaux. The faecal pellets of both species contain 5–10 μm sized carbonate particles. Whereas there is little else in Acmaea pellets, which are white, cylindrical and only loosely held together, chiton pellets include much fine manganiferous material, detrital quartz and broken diatom frustules, and are well bound with mucilage which makes them dirty brown-grey in colour and ovoid. The data indicate that physical processes need no longer be assumed to be the major factor in the recycling of carbonate from shells, and in their fragmentation.

Shell bioerosion in high-latitude low-energy environments: Firths of Clyde and Lorne, Scotland

Abstract The agents of bioerosion in the Firths of Clyde and Lorne include endolithic algae, Cliona , phoronids, Acmaea virginea, Lepidopleurus asellus and regular echinoids which directly and indirectly cause shell disintegration and the production of calcareous sediments. Algae and Acmaea are very effective in shell erosion, but are confined to the photic zone where over 90% of the shells have suffered from their attack. Other bioerosion agents are not depth-restricted but individually affect a lesser number of shells. The photic limit (approx. 30 m) is estimated from the disappearance of the deepest growing algae “Conchocelis” and Ostreobium . The density of algal boring is high due to the low energy at the bottom. It is shown that algal borings and Acmaea grazing are also important as palaeobathymetric tools.

Calcareous sediments on the nearshore continental shelf of western Scotland

The highest carbonate values are associated with exposed coasts of Lewisian gneiss, reaching 95% CaCO 3 in SW Tiree: around Iona average values are 80 to 85% CaCO 3 . Maximum values of 60 to 70% on Islay indicate a regional gradient which reflects a reduced fetch southwards, and a greater input from reworked glacial drift and fluvial material. High energy carbonate deposits are distinguished by the low diversity of their components, nearly all coming from fragmented molluscs and barnacles. Low energy carbonates have diverse components, with foram and echinoid values each reaching 10%. The ratio most easily separates the two facies (high energy mean = 18-5: low energy mean = 2-1). In the immediate offshore tidal-swept zones such as the Sound of Iona barnacles are the most important contributors to the sediment. Most of the carbonate is fresh and by its relationship to contemporary shore ecology can be shown to be of present-day origin. Offshore, however, relict grains are widespread. Their pitted surfaces are inhabited by very abundant diatoms, and it is tentatively suggested that local carbonate dissolution may be caused by them. Additionally they represent a much more abundant source of silica for eventual silicification of the carbonate than do sponge spicules, which are rare.

Recent dolomitic concretions of crustacean burrow origin from Loch Sunart, west coast of Scotland

ABSTRACT Dolomitic nodules were dredged from modern muds of 50 to 70 m water depths, from Loch Sunart on the west coast of Scotland. The nodules which are characterized by a central open passage which may branch and show a swelling at the branching points are believed to have formed around the burrows of thalassinidean crustaceans. After concretion formation the lithified burrows were exhumed and both their external and internal surfaces colonised by encrusters and borers. The concretions are analogous to Jurassic concretions described from Poland, and are the first such structures to be described from the Recent. Concretion formation is thought to be due to enhancement of carbonate precipitation by sulphate reducing bacteria concomitant with decay of organic material producing increased alkalinity. From an initial CaCO3 phase, addition of equal amounts of the ubiquitous Mg2+ ion plus CO32- from further sulphate reduction by bacterial action gives rise to the formation of dolomite (48 mole % MgCO3).

Carbonate-basaltic sediments from Cobb Seamount, Northeast Pacific: Zonation, bioerosion and petrology

Abstract Six dives were made with “Pisces IV” at depths of −30 to −700 m, and sediments collected by scoop. Bathymetrically zoned bioclastic carbonates rim the seamount. They reflect the zonation of the living fauna but are complicated by relic Mytilus gravels from the Wisconsin low-stand. Successive faunal zones contain prominent echinoderms: echinoid-bivalve-bryozoan (−30 to −90 m); crinoid-brachiopod (−90 to −180 m); coralbrachiopod-ophiuroid (−180 to −500 m); hydrocoral-crinoid-gorgonian-ophiuroid (−500 to −700 m). Sediments down to −360 m average over 80% CaCO3. Within the photic zone (down to −185 m) they are dominated by bivalves and bryozoans; below this, by benthic and planktonic foraminifera. Bioerosion occurs within the photic zone by algal boring and echinoid/limpet grazing, but at lower rates than during the low-stand. Wave base is −150 m. A conspicuous belt of ahermatypic reefs occurs on pillowed dykes at −310 to −360 m. Gorgonian coral bases perform a rapid secondary encrusting role, e.g. lining eunicid polychaete tubes associated with the coral Lophelia. Basaltic granules, hyalotuff of basaltic composition, feldspar crystal fragments and hyaloclastite grains dilute the carbonate, particularly in deeper water. Manganese nodules occur in the sample at −385 m. These nodules are small and could have formed in 20,000 yrs. High Mn:Fe ratios indicate their formation in an environment of high productivity. A poorly lithified hyalotuff beach, believed to have originated intertidally, is now seen at −310 m. Well-rounded basaltic granules occur down to −537 m and make up the bulk of the sediment at this depth.

An underwater television survey of facies variation on the inner Scottish shelf between Colonsay, Islay and Jura

Synopsis Using underwater television, eleven recurrent bottom facies have been identified and mapped over an area of about 250 km2. Bare rock areas are rare: Turritella-rich fine sands with trails, and crustacean-burrowed muds widespread. Coarse sands, fining northwards from megaripple fields in the Sound of Islay and the Passage of Oronsay are dominantly of biogenic origin. The kelp Laminaria saccharina was seen rooted in gravelly parts of the megaripple field in the former area (one of strong tidal currents but reduced wave action) where the calcareous alga Lithothamnium is an important contributor. Barnacles and molluscs dominate in the latter region, which is more exposed to Atlantic swell. Extensive spreads of Modiolus seem to rest on a tough clay surface, possibly of eroded late-Glacial material which also probably underlies Tarbert Bank, since it crops out along its eastern margin. The rippled sand body located on the Bank could represent residual out-wash. Facies analysis by combined underwater television and grab/dredge/box coring is optimal for shelf waters of intermediate depth (10–100 m): it reveals particularly well vagile epifauna, normally missed by conventional grabbing. In addition, it permits sediment/faunal samples to be studied in a truer perspective. Differentiation of facies by television scores over bottom sampling in higher energy, coarser sediments; but in lower energy muds and fine sands of the level bottom, where infauna dominate, biofacies analysis by faunal clustering and resin impregnation of internal structures will probably achieve greater refinement

Techniques for the Study of Fossil and Recent Traces

The study of trace fossils demands an acute observational technique because the structures must be visualized in three dimensions. In lithified rocks, a massive “structureless” bed should be regarded with suspicion; the homogenization created by intense burrowing can sometimes be revealed only in the laboratory. Techniques for enhancing obscure bioturbation structures differ but little from those used for physically formed sedimentary structures: initial sandblasting or staining may be followed by x-ray radiography and infrared photography. Thin sections of burrowed sediments should be slightly thicker than the normal 0.03 mm, to produce greater contrast; if of suitable areal dimensions (5×5 cm), they may be projected directly onto a screen by a slide projector.

Depth of deposition of Early Holocene raised sediments at Irvine deduced from algal borings in mollusc shells

Synopsis Micro-borings attributed to the chlorophytes Phaeophila and Eugomontia and the cyanophytes Hyella and Plectonema are recorded from shells in the early Holocene sediments of Irvine. The basal gravel beds generally show a higher density of algal borings and a greater percentage of algal-infested shells than the overlying sand unit. Borings of Eugomontia and the sponge Cliona and the calcareous algae Melobesia and Lithothamnium, are restricted to the basal gravel. Pockets of broken Lithothamnium observed in the sand unit are interpreted as reworked. By comparison with Recent micro-endolithic algal assemblages in the Firth of Clyde, it is concluded that the basal gravel formed in a low energy marine environment about 10 m deep: the overlying sands represent a true raised beach.