Abstract This study focuses on the recent evolution of a shallow-water dunefield system off La Pelosa Beach, located in the wind-dominated, microtidal, asymmetrical Asinelli Strait (Stintino, NW Sardinia, Italy). The system comprises four zones defined by a gradual decrease in current strength from the strait centre to more distal parts that form different bedforms. The strait centre (Zone A) is a substrate-carved channel passing downcurrent to a vast sandy shoal characterized by a subaqueous dunefield (Zone B) that is surrounded by a sandwave–ripple carpet (Zone C). The strait margin (Zone D) is dominated by a sandy cusp, the so-called La Pelosa Beach system. The extensive seagrass meadow located on the SE side of the strait-end zone feeds the system with bioclastic-rich sands. This carbonate sedimentary load is ensured by natural seagrass meadow retreats that are possibly boosted by human impact. The sediment transport is modulated by the interaction between two opposite currents triggered by the east-incoming Greco/Levante and NW-incoming Mistral winds. These currents experience a great variability due to multi-annual fluctuations (4–6 years) of the prevailing wind activities. Such cyclicity is responsible for periodic dune migration and and transport from La Pelosa Beach out of the system below the active part of the submerged beach (depth of closure). Bedform formations (ripples, dunes and sandwaves) and migrations within the strait are the result of complex physical processes observed in modern environments but rarely described in rock formations. The presented conceptual model can be used to recognize sediment ‘spillover’ processes on other modern, microtidal, wind-dominated straits, as well as proposing the main criteria for recognizing this strait sedimentary succession in the geological record.

During Middle and Upper Cambrian time central and eastern Montana and all of Wyoming formed a stable shelf bordered on the west by a geosyncline (miogeosyncline) trending through western Montana, southern Idaho, and northeastern Utah. This foreland area, the drowned western coastal plain of Laurentia and Siouxia, experienced its initial downwarping during the early half of the Middle Cambrian with the shore line finally stabilized across eastern Montana and Wyoming by Elrathiella zone time. A second major downwarping during the Dresbachian stage carried the shore line eastward into the Upper Mississippi Valley region. During all this time sedimentation was slow, often negligible over large areas; shoals and low islands were prevalent, especially in south-central Montana and western Wyoming; and in both coastal and off-shore parts of the shelf extensive sand and mud flats were frequently exposed or awash. The preserved fossils are predominantly trilobites and inarticulate brachiopods but probably 75 per cent of the Cambrian biotas were soft-bodied plants and animals, The great number and variety of tracks, trails, and borings in the sediments is in-, triguing evidence of the presence of soft-bodied forms, and a few impressions of jellyfish, graptolites, and sponges have been obtained. Most of the assemblages represent thanatocoenoses, piles of trilobite molts, and broken brachiopod shells swept together and deposited by waves and currents. Only the assemblages of the algal meadows (Collenia reefs) and the Eoorthis-Billingsella beds can be surely regarded as biocoenoses. The lithofacies as well as the fossils suggest, as might be expected over so large an area, that a variety of bottom conditions existed at any one time. Broad formational units of (1) a basal transgressive sandstone, (2) a succeeding shale, (3) a limestone with interbeds of shale, and (4) a regressive shale and limestone pebble conglomerate can be recognized for the Middle Cambrian over the entire region; in lithic detail each unit varies considerably. The basal sandstone may be missing; the limestone unit thins and wedges out in an easterly direction; and to the southeast, east, and northeast the entire section passes into a monotonous sequence of sands and shales carrying abundant evidences of shallow water deposition. During the earliest Upper Cambrian, conditions of sedimentation remained unchanged, and shales with intraformational pebble conglomerates constitute the lowest unit, succeeded by a fragmental oölitic limestone marking the major Dresbachian downwarping. The limestone wedges out eastward. The rest of the Upper Cambrian and basal Ordovician consists of a highly variable succession of sandy shales, calcareous shales, thin limestones, limestone pebble conglomerates, and dolomitic siltstones. The vertical and horizontal variability of these sediments indicates that the entire shelf area lay essentially at the level of equilibrium. No appreciable thickness of beds could be deposited, and the small amount of clastic material, the glauconite, and the limestone pebbles were reworked many times before final burial. From this lithic succession it is clear that the Middle-Upper Cambrian boundary as commonly recognized in this area is a facies boundary which is older in the west than in the east, rising in age across the shelf area; it does not correspond to the time boundary suggested by the fossils. The ecologic implications of the intraformational pebble conglomerates, the oölites, and the glauconite, all of which occur in striking abundance in the formations of the shelf area, are critically examined. Four types of pebble conglomerate are recognized, two of which are known to intergrade. Fragmentation by waves and currents is postulated; this may have occurred anywhere from exposed surfaces down to possibly 100 feet. Distinctions among the four types of conglomerate are based upon the amount of transportation after fragmentation and the probable depth of water in which final deposition occurred. Oölite is conspicuous in the fragmental limestones of Middle Cambrian and Dresbachian age and suggests deposition in a depth of 100 feet or less. Glauconite is absent from all transgressive sands or shales throughout the area, but it may appear directly above these beds and continue through the section becoming quite conspicuous in the higher beds of the Upper Cambrian. A compilation of information on glauconite permits these conclusions: (1) Several sites of autochthonous glauconite existed within this area. These sites are thought to have been broad swales, below the general level of the sea bottom, characterized by a very slow rate of sedimentation, a moderately anaerobic environment in the bottom sediments, and a large amount of putrefying organic matter. (2) For the sediments as a whole the presence of abundant glauconite suggests that they were derived from land areas where crystalline rocks were exposed and no large rivers reached the shore; the sea bottom supported a large and varied fauna, with a biota of scavengers and mud-eaters only in the broad swales. (3) The presence of glauconite in itself is not conclusive evidence of any definite water temperature or water depth. Five maps showing the general lithofacies distribution at five stages in Middle and Upper Cambrian time, and two paleogeographic maps are presented. A brief summary of early Ordovician depositional history and of post-Canadian pre-Upper Devonian tectonic and erosional histories is sketched in order to clarify the relations of the top of the Cambrian or basal Ordovician sequence of this area to the overlying beds.

DURING forty years devoted to collecting and studying graptolites, the writer has observed that their distribution with respect to rock facies and association with other fossils is distinctive. To a far greater extent than any other fossil, the graptolites are bound to one definite rock facies, and they tend to produce pure faunas. They have not been discovered in great numbers mixed with benthonic or nektonic fossils in sandstones and limestones. These two facts—restriction to the black shales and tendency to segregate themselves from other species—have repeatedly impressed themselves upon the writer and have caused him to wonder as to the fundamental meaning. The fact that most of the Paleozoic formations, regardless of their lithologic character, contain scattered individuals is dismissed with the interpretation that these are “stragglers.” They are greatly outnumbered by the non-planktonic forms. The significant fact is that domination of a fauna by graptolites amounts to exclusion of all except a few species, which also are characterized by an apparent aversion for any other rock facies, and which are so rare that active exploration and search of graptolite shales through the years has revealed only a comparatively small number. After surveying all the facts surrounding the occurrences of the graptolites and their associates, the writer is constrained to the opinion that these forms lived in sargasso-meadows of Paleozoic sargasso-seas in both the Atlantic and the Pacific oceans. Part I of this Memoir presents first the evidence for this conclusion. It is shown that benthos ruled . . .

The North American Great Basin is a useful venue for the study of dispersal, vicariance, and rates of molecular evolution among aquatic organisms because its Pleistocene hydrogeographic history is relatively well known. This study examines regional molecular variation in the amphipod Hyalella azteca using mitochondrial (mt) gene sequence (deoxyribonucleic acid [DNA]) data. Populations within several endorheic drainages in the southern Great Basin were analyzed to determine if they represent a monophyletic assemblage with respect to populations from the pluvial Lake Bonneville drainage in the northern Great Basin. We also tested whether the patterns of molecular diversification among populations in the southern Great Basin were consistent with a Pleistocene vicariance hypothesis, and if the magnitude of observed sequence divergence was concordant with standard molecular clock calibrations. Our results show that diversity and endemism among Hyalella populations in the southern Great Basin are high with respect to those in the Lake Bonneville Basin. We further demonstrate that hyalellid populations in the southern Great Basin are a polyphyletic assemblage with respect to their counterparts in the Bonneville Basin, suggesting that dispersal events have been partially responsible for the enigmatic relationships within this assemblage. The relationships among lineages within the southern Great Basin are largely enigmatic and are not concordant with Pleistocene hydrographic history. Our data also indicate that rates of molecular evolution have been heterogeneous; there is a 2.8-fold disparity in relative rates of mtDNA divergence among closely allied lineages. The magnitude of sequence divergence among lineages is inconsistent with standard molecular clock calibrations, and evidence indicates that accelerated rates of divergence may have contributed to the high diversity and endemism among Great Basin hyalellids, complicating reconstruction of the temporal sequence of biogeographic events.