Record of a sea level drop in the lower Mississippian limestones near Passo di Monte Croce Carnico (Carnic Alps, Italy)

Fluvial adjustment of the Lower Jordan River to a drop in the Dead Sea level

Contents vol. 25, 1988

Silurian sequence stratigraphy of the Carnic Alps, Austria

The “Lochkovian-Pragian Event” re-assessed: New data from the low latitude shelf of peri-Gondwana

During the latest Carboniferous and earliest Permian (Virgilian-Wolfcampian), the eastern margin of the Orogrande Basin in south-central New Mexico was rimmed by a narrow, tectonically unstable shelf, on which sediments of the Laborcita and Abo formations were deposited. Sediments of the Hueco Group accumulated on the more stable western margin of the Orogrande Basin. On the eastern shelf, the Laborcita Formation of the northern Sacramento Mountains represents a transition from marine to terrestrial facies and is composed of clastic-carbonate cycles. Limestones accumulated during relative sea-level highstands in a shallow marine shelf environment. Clastic sediments were deposited during relative lowstands when there was strong clastic influx in a nearshore to terrestrial environment. The overlying Abo Formation is composed of terrestrial red beds. On the western shelf, the Shalem Colony Formation of the Hueco Group, equivalent to the Laborcita Formation, is composed of mostly normal marine shallow shelf limestones with only minor interbedded clastics. The Robledo Mountains Formation and Hueco Group reflects a trend from a restricted shallow shelf and tidal flat clastics facies in the lower part to more open conditions in the upper part, which also prevailed during deposition of the overlying Apache Dam Formation. Limestones of the Laborcita Formation and Hueco Group contain smaller foraminifers, algae and problematic carbonate microfossils. All taxa except two pseudoalgae in open nomenclature ( Litostroma (?) sp. and “problematicum gen. 1”) are taxa already described. Compared with the Carnic Alps (Austria/Italy), the assemblages of smaller foraminifers of the Laborcita Formation and Shalem Colony Formation are very similar to those of the Auernig and Carnizza Formations (Auernig Group), and Lower Pseudoschwagerina Limestone (Rattendorf Group), indicating an Orenbugian (“Bursumian”) to Asselian age. Smaller foraminifers of the Robledo Mountains Formation allow correlation with the Grenzland Formation and Upper Pseudoschwagerina Limestone (Rattendorf Group) of the Carnic Alps, dated as Asselian to Sakmarian. Smaller foraminifers of the Apache Dam Formation suggest an Artinskian age, correlating with the Wolfcampian, or the Trogkofel Group of the Carnic Alps.

Cretaceous sandstones are oil and gas productive throughout a large area in the Denver basin. The Zenith field is a recently developed area that contains significant reserves in the D sandstone. Minor production also comes from the J sandstone. Detailed mapping of the D sandstone suggests that productive sandstones are of channel origin within a valley-fill complex. Trapping of petroleum appears to be mainly stratigraphic with structure playing a minor role. The stratigraphy of seven Cretaceous stratigraphic intervals was analyzed to determine if paleostructure may have influenced D depositional patterns. Thickness variations within stratigraphic intervals are caused by unconformities, convergence, and normal faulting. Thickness variations caused by unconformities and convergence may be related to paleostructure; variations caused by normal faulting are postdepositional and related to Laramide structure. Analyses of seven stratigraphic intervals clearly show that paleostructure influenced D sandstone depositional patterns. A new model proposed for D sandstone deposition incorporates paleotopography and sea level changes. During or immediately after the deposition of the Huntsman Shale and a thin regressive D sandstone deposit, a structura low area formed. The low area was probably created by basement fault-block movement. Concurrent with the tectonics, a drop in sea level occurs which drains a portion of the D depositional basin. A drainage system develops and follows the low area and incises through the regressive D deposit and into the Huntsman Shale. A sea level rise occurs and thick D channel sandstones are then deposited within the End_Page 949------------------------------ eroded valley. The present structure at the top of the J sandstone (stratigraphically older than the D sandstone) is a structural low in the area where D valley-fill sandstones occur. The trend and location of the low at J level are identical to the trend and location of the D valley-fill deposits. Thus, the present low at the J level confirms the paleostructure interpretation. This new model for D sandstone deposition, incorporating paleotopography and sea level changes, provides a new idea for petroleum exploration in the Denver basin. End_of_Article - Last_Page 950------------

An Alpine immigrant: Phragmoceras Broderip, 1839 from the Silurian of the Carnic Alps (Austria)

Detailed analysis of over 1,000 subsurface Silurian palynology samples from 34 wells has allowed the development of a robust biostratigraphy based on acritarchs, chitinozoans and cryptospores for the Qusaiba Member of the Qalibah Formation, central Saudi Arabia. The new index fossils described herein augment the Arabian Plate Silurian chitinozoan zonation. The high-resolution biostratigraphic zonation consists of nine First Downhole Occurrences (FDOs) from the lower Telychian through Aeronian. In particular, three regionally recognizable palynologic horizons were identified within the lower part of the informally designated Mid-Qusaiba Sandstone (Angochitina hemeri Interval Zone), and above the FDO of Sphaerochitina solutidina. This high level of biostratigraphic resolution provides a framework for the integration of the sedimentology and calibration with global sea level curves, leading to a detailed understanding of the sequence stratigraphic evolution of this part of the Silurian in Saudi Arabia. Sedimentological core studies identify three Depositional Facies Associations (DFAs) within the Mid-Qusaiba Sandstone interval, including: (1) shelfal deposits (DFA-I) characterized by interbedded hummocky cross-stratified sandstones, graded siltstones and bioturbated mudstones; (2) turbiditic deposits (DFA-II); and (3) an association of heavily contorted and re-sedimented sandstones, siltstones and mudstones (DFA-III) that is considered representative of oversteepened slopes upon the Qusaiba shelf. Integration of the newly recognized palynostratigraphic horizons and the sedimentological data facilitates an understanding of the sequence stratigraphic evolution of the Mid-Qusaiba Sandstone interval and its immediate precursors. Thus a Maximum Flooding Surface (MFS) is identified from significant palynostratigraphic, as well as sedimentological evidence, and concurs with the MFS identified regionally with the Monograptus convolutus Graptolite Zone. Several mud-prone cyclothems downlap onto the MFS. Each of these is identified by its own palynostratigraphic marker: these mud-prone cyclothems represent the distal parts of a Highstand Systems Tract (HST). The end of the HST is marked by evidence of a major, episodic drop in relative sea level. Thus, a relationship is identified wherein successive palynostratigraphic marker horizons, newly identified in this study, are partially eroded by the introduction of sandy turbidites (DFA-II). These turbidites arise from storm-induced erosion of gully complexes in the upper submarine slopes that are present as topography upon the Qusaiba shelf. Each of the successive drops in sea level is separated from the next by a minor, subsequent sea level rise, which precludes further submarine erosion and turbidite deposition, and is instead evident in the widespread occurrence of shallow marine (shelfal) muds and sandy tempestites (DFA-I). The lowstand per se is considered to be represented by the most widespread distribution of the DFA-II turbidite deposits, and is associated with the youngest Mid-Qusaiba Sandstone marker horizon identified in this study, namely Rugosphaera agglomerata n.sp. The youngest unit of DFA-II lowstand turbidites is limited in its occurrence to the more proximal parts of the study area, and thus is considered to represent the onset of the succeeding Transgressive Systems Tract (TST). Of the biostratigraphic indices used for correlation within the Qusaiba Member, Rugosphaera agglomerata and Eupoikilofusa curvata are formally described and two additional important species, Fractoricoronula n.sp. and ?Oppilatala n.sp., are retained in open nomenclature.

The fusulinacean faunal content of the Bombaso Fm. and lower part of the Auernig Group (Carnic Alps, Austria/Italy) is reviewed and completed by data on conodonts and algae. Four different faunal associations can be distinguished within this stratigraphic interval.

Givetian and lower Frasnian carbonates of pelagic carbonate-platform and distal slope-apron settings in the Harz Mountains of Germany (Herzyn Limestone Formation), the eastern Moroccan Central Massif (Ziar-Mrirt Nappe), and the Carnic Alps in Austria-Italy (Valentin and Pal Limestone Formation) show strong evidence for bottom-current activity during deposition. Calcarenites, laminated calcisiltites, and mottled calcisiltites and calcilutites can be distinguished, which are similar to recent calcareous bioclastic contourites. They combine faint structures caused by current action with pervasive bioturbation. Calcarenites are mostly represented by styliolinid grainstones to packstones with rarely preserved parallel lamination and ripple cross-lamination. Laminated calcisiltites are particularly rich in non-carbonate components with a higher density than calcite such as conodonts and phosphatic intraclasts. Relics of coarsening-upward to fining-upward micro-sequences a few centimetres thick are preserved in the Moroccan record. Erosional surfaces, hardgrounds and condensed phosphates are more typical of the Harz Mountains and the Carnic Alps. The bottom-current influenced facies build up strongly condensed and reduced sequences that occur at the same stratigraphic interval in different areas of central Europe and NW Africa. Variations in rate of accumulation, magnitude of erosion and microfacies, which are found across the three regions, are compatible with a contourite interpretation. The widespread current-induced reworking of calcareous sediments and phosphate formation during the Givetian and early Frasnian as well as the associated erosion marked by pronounced hiatuses all signal a major palaeocirculation event. Thermohaline currents were intensified by the acceleration of flows constricted in narrow oceanic passages between the approaching continental plates Laurussia and Gondwana. Areas affected were the southeastern Rhenish Sea shelf, which occupied the distal passive margin of Laurussia, the disintegrated northern continental margin of Gondwana, whose sedimentary record is now preserved in the Moroccan Meseta, and deep marginal plateaux of the Noric Terrane in the western part of the Prototethys. Thus, the occurrence of fossil calcareous contourites confirms a very advanced convergence between Gondwana and Laurussia and the minor terranes between during Middle and Late Devonian times.

The Lea Park-Belly River transition in central Alberta comprises a downlapping wedge of coastal sediments. The transition is characterized by numerous stacked shoreface successions that young in an eastward direction. They are very sharply based such that the transition between the mid/lower shoreface and the shelf sediments is either absent or very thin. The shoreface successions are laterally equivalent to incised fluvio-deltaic sediments and are interpreted to indicate forced regressions, where a relative drop in sea level causes the shoreline to move rapidly basinward. The most likely cause of these regressions is a change of subsidence rate due to active loading in the Cordillera acting in concert with a third-order relative drop in sea level. Subsidence due to tectonic loading would allow for the aggradation of coastal sediments. During periods of lesser subsidence or greater sea level drop, distributions would incise and the position of the shoreline would move rapidly basinward, resulting in the subsequent sharp-based shoreface succession. This may indicate that in foreland basins, where subsidence acts to cancel relative drops in sea level, sequence boundaries due to third-order drops in sea level may be replaced by a stacked wedge of sharply based coastal successions.

Drowning of algal mounds: records from the Upper Carboniferous Lower Pseudoschwagerina Limestone, Carnic Alps, Austria

We demonstrate in this paper that satellite altimeter data resolve the drop in the Aral Sea level during 1993–2000 of about 0.6 m per year resulting in a change of surface area from 35000 to 22000 km2 and volume from 270 to 130 km3. The sudden drop in the sea level of the Northern basin on 04.21.1999 resulting from dam break‐up is also clearly resolved. The temporal and spatial variability of sea level reveals response patterns which are characteristic for friction dominated shallow sea dynamics. The combination of salinity and sea‐level data enables to identify the major events of environmental transition, which are associated with the temporal variability in the total salt content: (1) a peak in 1993–1994, and (2) an increasing trend in the last decade. These events are indicative of an increase in the discharge of ground water, but could also reveal overestimated salinity estimates from recent observations.

The mid-late Holocene drop in relative sea level has resulted in the deposition and progradation of coastal facies along the Enjefa Beach in Kuwait, NW Arabian Gulf. These coastal facies display various depositional subenvironments, including landward foreshore beach, tidal channel, shallow tidal channel, and coastal sand flat. The foreshore beach facies consist of dominantly planar laminated beds. Main tidal channel subenvironments are interpreted for the trough cross-bedded facies. The shallow tidal channel deposits are composed of variable oriented planar wedge-shaped and ripple cross-bedded facies. The coastal sand flat deposits consist of extensively burrowed beds with a large network of Ophiomorpha burrows. These exposures are overlain by the recent continental deposits of rubbles and artificial infill. The mere exposed occurrence of these rocks as well as the above facies stacking pattern reflects progradation as a result of relative drop in sea level. Based on the carbon dating of these sediments, the last phase of the relative sea level drop likely occurred around 1,800 years ago. Neotectonics of local structuring, due to the regional northeastward movement of the Arabian Plate under the Zagros Fold Belt, might have a role in this sea receding

Carbonates and relative changes in sea level