摘要:

ABSTRACTThe anatomy of progradational gravel beaches was analysed in two different tectonic settings: the strongly subsiding Messinian foreland basin of the eastern Southern Alps and the rising Pleistocene marine terraces of the outer margin of the Apenninic chain. Progradation took place near the mouths of fan‐delta or braid‐delta distributaries debouching in microtidal landlocked basins (palaeo‐Adriatic and palaeo‐Ionian seas). A wind‐wave climate, comparable to that of the present‐day, was characterized by periodic intense storm activity. Most of the beachface progradation is thought to have taken place during the post‐storm recovery and fair‐weather stages, whereas the impact of storms is mostly recorded by the cutting of erosional surfaces on the beachface and by emplacement of poorly‐sorted and coarse‐grained gravels on the shoreface by storm‐intensified seaward‐trending flows and behind the highest berms by washover processes. Analysis of the sequences led to the identification of a number of typical divisions which are differently developed in the two settings compared here. The characteristics of the Apenninic sequences suggest a strongly variable wind‐wave climate, with periodic changes from a relatively dissipative barred morphology during storms, accompanied by strong longshore currents and rip development, to a more reflective state, typical of recovery and fair‐weather stages. The Southern Alpine sequences, on the other hand, are characterized by poor evidence of barred morphology and of longshore currents and widespread effects of shear‐ and gravity‐sorting in the lower‐beachface gravels. These features indicate a comparatively more reflective average state, due to location of beaches along the deeply embayed head of the palaeo‐Adriatic, and the very narrow directional width of the incoming wave spectrum, which was almost shore‐normal most of the time. In addition to well‐developed shape zonation of gravels, the common presence of wave‐generated gravel megaripples in the Southern Alpine sequences indicates the greater influence of long‐period waves, due to the greater available fetch distances. In both settings the cyclicity is thought to be geneti

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01737.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTSeveral well‐preserved Late Pleistocene spit systems occur uplifted in northern Jylland, Denmark. Their present‐day morphological expression allows detailed study of spit growth patterns while the internal sedimentological organisation can be examined in a series of pits distributed along the length of the spits. Two characteristic vertical sequences are recognized in the systems. The first (Sequence I) consists of a giant‐scale cross‐bedded foreset unit, overlain by topset and beach units, while the second (Sequence II) consists of the foreset unit overlain by bar‐trough and beach units. The two sequence types pass laterally into each other with a short overlap zone. They can be interpreted in terms of Meistrell's (1966, 1972) model for spit‐platform growth based on scaled wave tank experiments. The giant‐scale cross‐bedded unit corresponds to prograding of a coarse‐grained subaqueous spit‐platform while the topset, bar‐trough and beach units reflect the growth of the subaerial spit. The alternation between sequence I and II reflects the inversely related growth of the spit and platform structures: when the rate of subaqueous platform progradation declines, the subaerial spit grows uniformly, and when the platform progrades uniformly spit growth declines. The model is probably only valid for relatively coarse‐grained systems because only these deposits would have a relatively steep front. The water depth in which the spit system progrades and thus bottom topography, determines the thickness of the giant‐scale cross‐bedded foreset unit because the water depth over the top of the platform is relatively constant. If the water is less than a few metres deep the spit‐platform is not developed as seen where the Late Pleistocene spit systems prograded over elevations of the sea bottom. Conversely, the correct recognition of spit‐platform sequences allows precise determination of sea‐level and water depth at the time of formation. Finally, the model adds one further mode of formation of giant‐scale cross‐bedding to those already known from fluvial transverse, lateral and point bars, s

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01738.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe Algodones dune field of southeastern California is one of the largest active dune fields in North America. The dune field is migrating in an easterly direction, oblique to the resultant sand flow direction (S 24° E). The migration of the Algodones results from an interaction between regional winds and the dune field. This interaction generates a localized secondary flow that has caused the dune field to migrate in a direction oblique to the resultant sand flow direction.Four lines of evidence suggest that the Algodones has migrated in an easterly direction: (1) A ramp, interpreted as the trailing edge of the dune field, 35 m thick and 500 m wide composed of aeolian deposits that borders the western edge of the dune field. No similar deposits are found on the eastern (leading edge) margin of the dune field. (2) Leading‐edge sand‐sheet deposits are exposed in interdune areas within the dune field. These deposits are west of the modern leading‐edge sand sheet. (3) Across the breadth of the dune field sands are consistently coarser and more poorly sorted in the west and finer and better sorted in the east. This observation suggests that sand is transported from west to east. (4) Eastward migration of a large compound‐complex crescentic dune.If the dune field continues to migrate it will deposit a vertical sequence consisting of: a basal sand‐sheet deposit consisting of wind and water‐ripple laminae, small‐scale aeolian cross‐strata, and ephemeral stream (wadi) deposits; aeolian dune deposits consisting of medium‐scale aeolian compound cross‐strata; small‐scale simple sets of aeolian cross‐strata with highly variable dip directions; a sand sheet containing low‐angle wind‐ripple cross‐strata capped by a coarse s

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01739.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTLate Miocene volcaniclastics of the Ellensburg Formation (Washington, northwestern USA) are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a Cascade Range source area. Two facies associations are recognized. The first is composed of laterally restricted, well‐sorted, polymictic conglomerates representing a gravel‐bedload regime during inter‐eruption periods. The second comprises laterally extensive sheets, bounded by deeply‐developed paleosols, composed of monomictic pebbly, pumiceous dacitic sandstones with intercalated debris‐flow and hyperconcentrated flood‐flow deposits. These sheets aggraded in response to eruptive events that are sometimes recorded by air‐fall tephras at the bases of sequences. Debris‐flow and hyperconcentrated flood‐flow deposits occur as far as 120 km from source, but are uncommon beyond 50 km. Hyperconcentrated flood‐flow deposits initially increase in abundance away from source as debris‐flow deposits diminish, suggesting formation of hyperconcentrated flood flows by dilution of debris flows. Sandy facies form broad sheets dominated by scour‐fill bedding, in proximal and medial settings, and grade to narrower, trough cross‐bedded sheets in distal settings, suggesting moderation of flood discharges with distance. Base‐level changes associated with episodic sediment influx caused incision‐aggradation cycles in mainstream settings, and episodic impoundment of tributaries to form lakes or rapidly aggrading, poorly drained floodplains. Although volcanism was the primary control on depositional style, concurrent development of the Yakima fold belt produced a structure‐consequent drainage pattern that determined sediment dispersal, and basin subsidence permitted preservation of both syn‐eruption and inter‐eruption facies. Detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics, but dominates deposits of small tributary streams flowing tow

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01740.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe early Pliocene Shirahama Limestone is a grainstone‐packstone principally composed of fragments of algae, bryozoa, and echinoderm and subordinate volcanic rocks. The limestone was variously dolomitized and the regional distribution of dolomite is patchy. Dolomite occurs as isolated crystals filling pores, moulds, and solution vugs, and mosaic aggregates replacing bioclasts. Calcite occurs as rim and pore‐filling sparry cements, and as calcareous skeletons. Isotopically, the dolomites are classified into a heavy oxygen group (−2 to − 3.5%0PDB) and a light oxygen group (−5.5 to − 7.5%0PDB). Calcite associated with heavy oxygen dolomite has δ18O of − 6.5 to −8.5%0PDB, whereas those associated with light oxygen dolomite have a wide range from −7.5 to −14%0PDB. Calcite in dolomite‐free limestone has an oxygen isotopic composition of −2 to −8.5%0PDB. Textures, chemistry, and isotopic evidence indicate that heavy oxygen calcite formed in freshwater, and heavy oxygen dolomite in a meteoric‐marine mixture of 10–30% seawater. Light oxygen calcite and dolomite precipitated from modified hydrothermal fluids at approximately 30–65°C.Petrographic features, and both isotopic and chemical evidence suggest that the Shirahama Limestone was exposed to freshwater soon after deposition. Subsequently blocky calcite precipitated (Stage I). The limestone was locally submerged in the meteoric‐marine mixture due to gradual subsidence or eustatic movement. This led to the precipitation of heavy oxygen, zoned dolomite and dolospar (Stage II). Hydrothermal alterations occurred in the area a few Myr ago, and related hydrothermal fluids and mixed meteoric‐hydrothermal waters caused dedolomitization of some zoned dolomite, partial dissolution of vuggy dolomite, precipitation of limpid dolomite and recrystallization of some earlier dolomites (Stage III). Zeolites were also precipitated from these fluids. Finally, the Shirahama Limestone was exposed again to freshwater and sparry calcite precipitated to plug s

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01741.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01742.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY