Upper Part Of Unit Research Articles

Giant polygons in the Realmonte Mine (Agrigento, Sicily); evidence for the desiccation of a Messinian halite basin

ABSTRACT The Messinian Realmonte salt deposit consists of a 400-600 m-thick halite succession that can be divided into four main lith ologic units (base to top): A) cumulates of plates settled out from a stratified water column; B) plate cumulates in a shallowing-upward sequence containing six kainite layers near the top of the unit; C) cu mulates of skeletal hoppers with chevron overgrowths precipitated from a nonstratified water body; and D) skeletal halite and anhydrite. Spectacular vertical fissures cut through the upper part of unit B at its boundary with salt unit C. These fissures are spaced at intervals of up to 5 m apart, extend down to a depth of 6 m, and are filled by red mud. The salt beds affected by the fissures are commonly upturned (buckled) and are truncated and onlapped by the succeeding flat-lying halite beds of unit C. Additionally, the upturned layers are cut by vertical dissolution pipes that extend down to as much as a depth of 4 m below the fissured surface. To explain the observed superimposed structures we propose two mechanisms, both induced by the desiccation of the evaporite basin: (1) When the salt surface was exposed, the halite layers were buck led, broke into polygonal crusts, and formed tepee structures. The buckling was caused by a net volume increase due to thermal expan sion, and what is more important, precipitation of new halite induced by evaporative pumping of brines moving up from the groundwater table. The buckled salt layers were then affected by meteoric dissolu tion, which created vertical pipes. (2) The tepee structures and the associated zone of dissolution pipes are further crosscut by a new network of large polygons. These new polygonal sets are outlined by deep fissures in which red silt collected, blown by the wind or carried by episodes of surface flooding. This type of deep contraction crack is known to be produced by volumetric changes induced by annual temperature fluctuations. The documentation of contraction polygons and other exposure fea tures represents the first report of desiccation events during the de position of the Sicilian salt. No geological data are yet available to settle the questions if the salt of Sicily is correlatable with the halite present beneath the floor of the Mediterranean and if the desiccation of the Realmonte salt basin could have been induced by: a) local tectonic causes (uplift of the basin floor by thrust activity, b) simple evaporitic drawdown, c) basinwide drop of the Mediterranean sea level, or d) complex interaction among these factors.

The 15 September 1991 pyroclastic flows at Unzen Volcano (Japan): a flow model for associated ash-cloud surges

Large-scale collapse of a dacite dome in the late afternoon of 15 September 1991 generated a series of pyroclastic-flow events at Unzen Volcano. Pyroclastic flows with a volume of 1×10 6 m 3 (as DRE) descended the northeastern slope of the volcano, changing their courses to the southeast due to topographic control. After they exited a narrow gorge, an ash-cloud surge rushed straight ahead, detaching the main body of the flow that turned and followed the topographic lows to the east. The surge swept the Kita-Kamikoba area, which had been devastated by the previous pyroclastic-flow events, and transported a car as far as 120 m. Following detachment, the surge lost its force after it moved several hundred meters, but maintained a high temperature. The deposits consist of a bottom layer of better-sorted ash (unit 1), a thick layer of block and ash (unit 2), and a thin top layer of fall-out ash (unit 3). Unit 2 overlies unit 1 with an erosional contact. The upper part of unit 2 grades into better-sorted ash. At distal block-and-ash flow deposits, the bottom part of unit 2 also consists of better-sorted ash, and the contact with the unit 1 deposits becomes ambiguous. Video footage of cascading pyroclastic flows during the 1991–1995 eruption, traveling over surfaces without any topographic barriers, revealed that lobes of ash cloud protruded intermittently from the moving head and sides, and that these lobes surged ahead on the ground surface. This fact, together with the inspection by helicopter shortly after the events, suggests that the protruded lobes consisted of better-sorted ash, and resulted in the deposits of unit 1. The highest ash-cloud plume at the Oshigadani valley exit, and the thickest deposition of fall-out ash over Kita-Kamikoba and Ohnokoba, indicate that abundant ash was also produced when the flow passed through a narrow gorge. In the model presented here, the ash clouds from the pyroclastic flows were composed of a basal turbulent current of high concentration (main body), an overriding and intermediate fluidization zone, and an overlying dilute cloud. Release of pressurized gas in lava block pores, due to collisions among blocks and the resulting upward current, caused a zone of fluidization just above the main body. The mixture of gas and ash sorted in the fluidization zone moved ahead and to the side of the main body as a gravitational current, where the ash was deposited as surge deposits. The main body, which had high internal friction and shear near its base, then overran the surge deposits, partially eroding them. When the upward current of gas (fluidization) waned, better-sorted ash suspended in the fluidization zone was deposited on block-and-ash deposits. In the distal places of block-and-ash deposits, unit 2 probably was deposited in non-turbulent fashion without any erosion of the underlying layer (unit 1).

High-frequency sea-level fluctuations and plant habitats in Cenomanian fluvial to estuarine succession: Pecínov quarry, Bohemia

Abstract The succession of the Peruc and Korycany Members in the Pecinov quarry, Bohemia, reflects the interplay of 4th-order sea-level fluctuations, of approximately 100–120 ka periodicity, and a long-term (3rd-order) sea-level rise of the late middle through early late Cenomanian. The succession studied includes deposits of a shallow, gravelly braided river (unit 1), tide-influenced braided river (unit 2), supratidal marsh (unit 3), tidal flat (unit 4) and ebb-tidal delta to estuary mouth fill (unit 5). Units 1–5 are interpreted as parasequences of a single depositional sequence; units 1–4 together make up a transgressive systems tract and unit 5 is a highstand deposit, capped by a sequence boundary. The presence of deciduous plants attests to a certain degree of seasonality of the climate. The mixing of thermophile and temperate vegetation, together with the presumed palaeolatitudinal position of the Bohemian Massif, suggests a generally slightly semi-arid (i.e., seasonally wet) regional climatic conditions during the deposition of units 1–5; substrates in local habitats in alluvial valleys and estuary margins were damp due to high local water table. The plant habitat of the alluvial valley (units 1 and 2) was characterized by the Myrtophyllum assemblage, which included an angiosperm-dominated gallery forest and shrubs on braid bars and islands. The transition between fluvial and coastal environments was characterized by a taxodioid backswamp forest (the Ceratostrobus assemblage). Coastal vegetation (i.e., the marsh habitats of unit 3 and the upper part of unit 5), changes distinctly between the two units, although both represent similar facies of high intertidal to supratidal origin. The Frenelopsis assemblage of unit 3, representing shrubby marsh vegetation growing seaward of the Ceratostrobus forest, differs strongly from the more xerophytic Sphenolepis assemblage of unit 5, characterized by herbaceous pteridophytes growing in the marsh in front of a coastal taxodioid forest. We explain the differences between units 3 and 5 by the different sequence-stratigraphic position of the two units: a high groundwater table and eutrophic substrate of the transgressive systems tract (unit 3) compared with a lower or falling regional water table and oligotrophic substrate during the late highstand systems tract (unit 5). Thus, temporal changes in coastal plant assemblages of the Pecinov section do not merely reflect lateral shifts of facies belts, but show that similar depositional environments in different systems tracts hosted very different floras.

Applications of Geochemical Data to Modelling Sediment Dispersal Patterns in Distal Turbidites: Late Quaternary of the Madeira Abyssal Plain

ABSTRACT An extensive geochemical database has been used to constrain depositional processes, sediment dispersal patterns and palaeoflow pathways of late Quaternary distal turbidites deposited on the Madeira Abyssal Plain (MAP). Turbidites are meter-thick, mud-dominated units with distinctive litho-, bio- and chemostratigraphic characteristics which have enabled the correlation of individual beds over > 67,000 km2. Geochemical profiles for Si, Ti, Al, Mg, Fe, Ca and Zr have been constructed for a representative 5-meter bed (turbidite b) at 18 core localities distributed across the central part of the MAP. These profiles, combined with grain-size and mineralogical data, demonstrate that turbidites consist of a 1-120 cm, laminated, sand/silt basal layer, rich in volcanic gla s, basalt clasts, heavy minerals and foraminifera (Unit 1), overlain by 1-5 m of poorly graded silty muds. Geochemical data additionally indicate the presence of a central layer which is relatively enriched in heavy mineral and flocculated clay-aggregate muds and is typified by high Ti, Fe, Mg and Zr contents (Unit 2). This unit is overlain by sediments containing increasing proportions of calcareous nannofossils and clay minerals (Unit 3) and characterized by enhanced Ca and Al concentrations. Unit 1 thins and fines, whereas Units 2 and 3 thicken and fine toward the deepest part of the basin and into local depressions and cul-de-sacs. It is argued that the majority of the turbidites were deposited from evolving highly concentrated flows, which prevented significant grain-size segregatio and enabled the rapid deposition of meter-thick poorly graded muds. The sorted and graded basal silts (Unit 1) were deposited primarily by an initially turbulent flow, which decelerated and matured into a highly concentrated fluid mud turbidity current. Shear at the flow base was responsible for the grading in the upper part of Unit I and in the silty muds of Unit 2. Fluid entrained under the current head, which migrated up through the flow, carried light carbonate and clay minerals toward the turbidite top, leaving a central plug of dense muds overlying sheared sands/silts and silty muds. Isocon maps (produced by joining points of equal chemical concentration) based on mean values for Ca, Ti, Mg and Zr in Unit 2 are used to reconstruct sediment dispersal patterns and palaeoflow pathway . These maps confirm that turbidite b was deposited by turbidity currents which flowed axially southwestward across the central part of the MAP, implying that the Canary Islands and Madeira offer a potential source of sediment for MAP turbidites.

Late Proterozoic glacially controlled shelf sequences in western Mali (west Africa)

The Late Proterozoic deposits of the Bakoye Group (500 m) in western Mali constitute a remarkable example of a glacially influenced sedimentary record on an epicratonic platform. They are composed of alternating marine and continental formations which represent accumulation in a basin located in the vicinity of upland areas covered by ice sheets. One of these formations (the Ba4 Formation), which is the focus of this study, is composed of three major units. The basal Unit 1 is made up of carbonaceous coarse to fine grained sandstones which are organized in fining upward sequences and which comprise lenticular diamictite intercalations. This Unit is considered to represent the fore slope gravity flows of a subaqueous ice-cootact fan fed by meltwater streams (⪡glacioturbidites⪢). Unit 2 is made up of coarse to fine grained sandstones in a highly variable association of facies. This Unit is characterized by the abundance of wave ripples associated with convolute beddings. planar or wavy beddings and tabular or hummocky crossbeddings in a general shallowing upward trend. It also comprises evidence of gravity processes including debris flows and large slumped sandstone bodies. Unit 2 represents the progressive filling of the Ba4 basin and reflects the combined effect of glacially induced eustatism and isostacy during a phase of glacial retreat. The basal part of Unit 3 is made up of a succession (a few meters thick) of conglomerates, diamictites, sandstones, siltstones or carbonates lying on an erosional unconformity marked by periglacial frost wedges. The upper part of Unit 3 is thicker (100–150 m) and onlaps on these basal facies with a succession of sandstone bars exhibiting swaley and hummocky crossbeddings, large cut and fill structures, and planar laminations. Unit 3 is strongly transgressive, the lower shoreface and backshore deposits include algal mats and are onlapped by sand ridges emplaced in a high energy upper to middle shoreface environment. Overall, in tems of sequential stratigraphy, the deposits of the galcial Bakoya Groupe are organized in shallowing upward sequences limited by maximum flooding surfaces of regional extent. One of these sequences (the Ba4 Formation) is subdivided into parasequences generated in relation with local ice sheet fluctuation including glacioisostacy.

Carbonate Sedimentation in a Late Precambrian Shelf Sea, Victoria Island, Canadian Arctic Archipelago

ABSTRACT The Reynolds Point Formation is the second lowest unit in the Late Precambrian Shaler Group of Victoria Island in the western part of the Canadian Arctic Archipelago. It has a maximum known thickness of about 1,100 m. The formation may be divided into four members. The lowest member is made up of shale followed by a sandstone-dominated sequence. The overlying lower carbonate member is thicker than the others. It is overlain by a second terrigenous sandstone unit (mainly tidalites) that is succeeded by the topmost, carbonate-dominated member. The lower carbonate member may be considered as three units. Unit I (the lowest) is about 100 m of fine-grained shaly carbonate with sole, and ball and pillow structures, together with some thick and extensive stromatolitic banks. Unit I is interp eted as representing minor subsidence of the depositional basin, followed by gradual infilling and stabilization. Unit 2 is about 100 m of shaly nodular dolostone and limestone that was probably deposited below wave base in a lagoonal system protected by a barrier complex. A thin purple shale in this unit is an excellent stratigraphic marker. Unit 3 (450 m) consists mainly of interbedded oolites (commonly associated with stromatolites) and shaly carbonates. This unit was deposited under a warm (dry?) climatic regime, adjacent to a continental area of low relief in a depositional setting comparable in some ways to that of the Persian Gulf The oolites probably formed on tidal bars or in tidal deltas from which they were spread into topographic lows that normally received fine-grained muddy carbonate sediment. The upper part of Unit 3 includes some thin cross-bedded fine-grained quartzarenites which were mainly deposited as subtidal sand sheets. Hematitic oolites in the northern part of central Victoria Island are interpreted as an early diagenetic replacement phenomenon. Molar-tooth structure is common in shaly carbonates interbedded with oolites of Unit 3. The molar-tooth is interpreted as being due to early dewatering of muddy carbonate sediment, controlled in part by interbedding of very permeable sediment (oolite) with water-saturated mud. Carbonate sedimentation was interrupted by the influx of fine silt and sand (upper clastic member; 130 m thick) that spread over the depositional area (from the east?) and was deposited mainly from tidal currents. A return to carbonate deposition (upper carbonate member; 200 m thick) was followed by the evaporitic-red bed sequence of the Minto Inlet Formation.