John E. Sanders

Evidence of Shoreface Retreat and In-Place “Drowning” During Holocene Submergence of Barriers, Shelf off Fire Island, New York

At different times within the Holocene period, the barriers on the shelf off Fire Island, Long Island, New York, have responded to submergence through the contrasting processes of in-place drowning and landward retreat. In-place drowning is indicated by evidence of a relict shoreline 7 km seaward of the present beach at a depth of −24 m. Sedimentologic criteria for locating this inferred relict shoreline consist of characteristic relict shoreface sediments on the seaward side and of characteristic relict backbarrier sediments on the landward side of a lenticular belt of sand from which no samples are available. Based on published submergence curves, this inferred relict shoreline is tentatively dated at 8,500 to 9,000 yr B.P. At about 7,500 yr B.P., the breaker zone is inferred to have “jumped” 5 km landward from this relict barrier to form a new chain of barriers 2 km seaward of the modern shore at a depth of −16 m. Two cores collected seaward of the present beach in 14 to 16 m of water contain backbarrier salt-marsh peat which has been dated at 7,750 ± 125 and 7,585 ± 125 radiocarbon years. The peat underlies offshore sand which forms part of the shoreface of the modern barrier. These cores are evidence that the −16-m barrier migrated continuously landward and eventually became the modern barrier. Inlet-filling sands can serve as indicators of former locations of barriers and as criteria for determining whether barriers have been drowned in place or have migrated landward. If a barrier migrates continuously landward, it should leave behind a blanket of inlet-filling sands. If a barrier drowns in place, inlet-filling sands should form only narrow, linear lenses parallel to the shore.

SYMMICTITE: A NAME FOR NONSORTED TERRIGENOUS SEDIMENTARY ROCKS THAT CONTAIN A WIDE RANGE OF PARTICLE SIZES

Existing terminology of till-like sedimentary deposits tends to emphasize genesis without considering all possible modes of origin and to suggest relationships to tillite that may be erroneous. Therefore new terms ( symmicton for sediment; symmictite, for lithified equivalents) are proposed as general names for essentially nonsorted, noncalcareous, terrigenous deposits composed of sand and/or larger particles in a muddy matrix.

Chapter 6 Origin and Occurrence of Dolostones1

Summary This chapter begins with a historical review of the dolomite literature and traces progressive advances in the search for a solution of the “dolomite problem”. The “dolomite question” defied solution until the recent advent of the X-ray revolution, when dolomite identification became simple, fast, and efficient. The general framework of the “question” now appears to have been largely resolved as a result of intensive field studies of Quaternary carbonate sediments and the application of X-ray and stable isotope (carbon and oxygen) analysis to both Quaternary carbonate sediments and ancient limestones and dolostones. Many problems, however, are still outstanding, but they are on a different level. In the opinion of the authors, most dolostone deposits in the geologic record owe their origin to hypersaline brines. They must, therefore, be considered as evaporite deposits. Exceptions are uncommon and include those that are related to soil-forming and bacterial processes or have resulted from recycling of pre-existing dolomite. Hypersaline brines are formed by “capillary concentration” or by “refluxion” in the depositional environment in areas where evaporation exceeds precipitation plus run-off and by as yet unexplained processes in the subsurface. In “capillary concentration”, interstitial waters in the sediments transpire upward through porous sea-marginal sediments and evaporate at the sediment-air interface, a process similar to that which is responsible for caliche formation. This process leads to dolomite formation in supratidal and intertidal environments on broad shallow shelves with interfingering gypsum and/or anhydrite (landward) and marine carbonates (seaward). Under conditions of humid climate, however, anhydrite and gypsum may not develop. Supratidal to intertidal dolostones display many or most of the following structures: mud cracks, “birdseye” structures, burrow mottles, boundinage-like structures, scour-and-fill structures, channels, ripple marks, cross-stratification, and algal structures; these dolostones are commonly unfossiliferous and laminated and are texturally structureless or pellet muds. In “refluxion”, evaporation increases the concentration and density of the water in a restricted lagoon or intermontane basin. This produces a brine that sinks and migrates to the lowest possible topographic depressions where it may seep slowly through the underlying sediments, which are progressively dolomitized. Alternately in refluxion, dolomite may form directly from the brine with aragonite as a possible transitional phase, and with the deposition of layered dolomite mud at the bottom of the basin. The Mg/Ca ratio of the brine must be increased from that of sea water to a ratio larger than that which should be in equilibrium with both calcite and dolomite for dolomite to form. This ratio can be raised by the removal of calcium from the brine to form aragonite or gypsum or both; however, gypsum is only deposited in very shallow water where a supply of oxygen is ample, because it tends to be degraded to HzS and iron sulfide by bacteria where the oxygen supply is less. Hence layered dolomite formed under these conditions is indicative of a deeper water environment, of unspecified depth, with gypsum deposited in correlative positions along shore. Precaution is advised, therefore, in interpreting all syngenetic dolomite of supratidal origin. Both “capillary concentration” and “refluxion” may operate in the same basin; thus, at Salt Flat Graben, West Texas, an intermontane basin, the former is responsible for dolomite formation along the margin of the basin and the latter, in its center. Dolomite formed after burial, which includes most diagenetic and all epigenetic (fault and fracture-related) dolomite, owes its origin to subsurface brines. The origin of these brines is unknown, but their salinity is very close to that under which dolomite is formed in the depositional environment. Subsurface waters are normally depleted with respect to both magnesium and sulfate ions which were withdrawn from the brines during dolomitization. Dolostones occur in a variety of stratigraphic associations which can be summarized in the following main types: (1) Syngenetic dolomite, a dolomite which is formed penecontemporaneously in its environment of deposition as a micrite or as fine-grained crystals, and which may also: (a) interfinger with both marine and nonmarine evaporites, with or without associated terrigenous sediments; (b) interfinger with limestones, both marine and nonmarine, with or without terrigenous sediments; (c) interfinger with terrigenous sediments; (d) be in the form of dolomite crystals disseminated in terrigenous sediments; (e) be formed by biological agents, such as bacteria; and (f) occur in nonmarine environments as a weathering product in soils and caliche. (2) Detrital dolostone and detrital dolomite is formed by recycling of preexisting dolomite sediments. (3) Diagenetic dolostone is formed by replacement of limestone following consolidation of the sediment or coincident with the consolidation; this type of dolostone may also form by penecontemporaneous replacement, Diagenetic dolostone may form within individual beds or along surfaces of stratigraphic dis-continuities. (4) Epigenetic dolostone is formed by replacement of limestones with the replacement localized by post-depositional structural elements, such as faults and fractures. Most dolostones are formed by repIacement of pre-existing calcium carbonate sediments, but dolostones may revert back to calcium carbonate rocks by the process of dedolomitization.

Storm-graded layers on inner continental shelves: Examples from southern Brazil and the Atlantic coast of the Central United States

Abstract Size-graded layers have been reported from the United States and Brazilian Atlantic continental shelves, amid a bedform association that includes ripples, megaripples, sand waves, and sand ridges. Graded layers are most apparent in the lag gravels and shell hashes that overlie an older substrate in the troughs between sand ridges, but also occur as thinner, isolated beds within the sand ridges. At least four different models seem to be applicable to graded bed formation. The stratigraphic graded model, grading occurs within and immediately above the basal lag gravel of the Holocene transgressive sand sheet. It is a consequence of the generation of the sand sheet by erosional shoreface retreat. Such graded sequences are formed by landward facies displacement over thousands of years, but the process is storm-mediated, and sequences may contain thinner storm-graded layers. The storm-graded model calls for entrainment of sediment by storm flow and subsequent graded deposition under waning energy. In the bedform migration model, the migration of megaripples and sand waves may produce thin graded beds at any level within the Holocene sand sheet, and sand ridge migration can cause graded sequences in the basal part of the Holocene sand sheet. The bottom liquefaction model relies on storm-wave pumping of the seafloor sands with their resultant liquefaction and settling of coarser clasts. Beds in which sorting is confined to coarse shell material may be of this origin. These models are believed to be complementary rather than mutually exclusive.

Barrier island evolution in response to sea level rise; discussion and reply

ABSTRACT During sea-level rise, barrier islands are forced landward with shoreface retreat. While retreat rate is dependent upon the balance between sediment supply and sea-level change, continuous migration, probably intermittent at times, as opposed to barrier overstepping, is clearly favored for low coastal plain barriers. Barrier drowning and subsequent surf-zone skipping are theoretically possible, but evidence to date from the New York shelf is not convincing. Barrier islands along the U.S. Atlantic and Gulf Coasts can be expected to migrate continuously landward with sea-level rise. Exhumation of pre-Holocene or partially consolidated sediments on the shelf surface with transgression can minimize but not prohibit barrier retreat.

Concepts of fluid mechanics provided by primary sedimentary structures

ABSTRACT Primary sedimentary structures can be used to infer conditions of flow which prevailed within fluid currents which deposited the sediment. Three particularly important conclusions about fluid flow conditions can be drawn from primary sedimentary structures: 1) The mechanisms of suspension and traction, as defined by G. K. Gilbert in 1914, are fundamentally different, and can be recognized by the different gross structures which they produce in sediments, and by the different effects which are brought about by the distinctive paths followed by particles which move under the influence of each mechanism; 2) The effects of grain-to-grain encounters in cohesionless sediment, which arise in response to current flow, can also be recognized in primary sedimentary structures, as can analogous ffects in cohesive sediments; and 3) Mass shearing effects at the fluid-sediment interface may produce deformation in cohesive sediments. The interpretations of primary sedimentary structures summarized herein suggest new lines of experimental investigation of fluids and sediments, in which attempts should be made to duplicate some of the mass shearing effects which have been recognized in cohesive sediments found in nature.

A field technique for making epoxy relief-peels in sandy sediments saturated with saltwater

ABSTRACT A quick-setting epoxy mixture is available that enables relief-peels to be made in the field for revealing primary sedimentary structures in sandy sediments. The mixture consists of five parts each of CIBA Epoxy #6005 and CIBA Epoxy #6010, three parts of CIBA Hardener #850, and one part of CIBA Hardener #830. Polymerization generally occurs within 1 hours and is not inhibited by either freshwater or saltwater. All additional four hours is required for curing. Depth of impregnation is controlled by sediment texture, sorting, and permeability. Moisture and low temperatures inhibit polymerization; the method is not recommended for temperatures below 45°F.

Chemical Changes in Interstitial Waters from Continental Shelf Sediments

The chemical characteristics of interstitial water from cores from the inner and outer shelf off Long Island are compared with the overlying sea water. Cores from the inner shelf consist of clean sands, whereas those from the outer shelf contain a mud fraction. [Various chemical ratios are determined, as are the Eh-pH relations.] The decrease in pH and Eh below the water/sediment interface is attributed to the activity of anaerobic bacteria, and the reasons for other chemical changes are not known but are thought to result from diagenetic changes within the sediment.

BALD MOUNTAIN LIMESTONE, NEW YORK: NEW FACTS AND INTERPRETATIONS RELATIVE TO TACONIC GEOLOGY

The “Bald Mountain limestone” of Ruedemann has been found to consist of the upper part of the autochthonous carbonate-rock sequence of the Champlain-Hudson valleys and not to be part of the Taconic sequence. At Bald Mountain this carbonate-rock sequence is overlain unconformably by the Snake Hill shale, with local basal limestone-pebble conglomerate (Rysedorph Hill conglomerate of Ruedemann). The Taconic thrust is present; along it the fossiliferous Lower Cambrian and younger fine-grained terrigenous rocks of the allochthonous sequence have been thrust over the autochthonous Middle Ordovician graptolite-bearing black shale.

Microtopography of Five Small Areas of the Continental Shelf by Side-Scanning Sonar

The microtopography of the continental shelf in five areas between Nova Scotia and New York was investigated with side-scanning sonar. The results were checked against those previously obtained by conventional methods in all areas and against later visual observations of the bottom from a research submersible vessel in two areas. Side-scanning sonar proved to be an ideal device for learning the distribution and relationships of rock, boulder, and sand bottoms and for measuring the patterns and trends of several sizes of sand waves and of large ripple marks.