Where: Italy (45.9° N, 12.0° E: paleocoordinates 1.6° S, 26.2° E)
• coordinate based on nearby landmark
• small collection-level geographic resolution
When: Val Gardena Sandstone Formation, Permian (298.9 - 252.2 Ma)
• Val Gardena sandstone and the Bellerophon Formation ( Upper Permian ) in the Dolomites and Carnia show an overall transgressive trend and record the transition from continental red-bed to marine sedimentation in an extensional tectonic setting. The eastward progression of the Bellephron transgression resulted in the diachrony of the boundary between Val Gardena Sandstone and Bellephron Formation. The transgression was actually punctuated by a series of cyclical pulsations, which resulted in complex interfingering of terrigenous, evaporitic and carbonate deposits and in the subdivisions of the sedimentary succession into a number of sequences. The various vacies are essentially contempareous and follow one another along a paleoslope gently inclined towards the east, so that the red beds grade basinwards (i.e. eastwards ) through sabkha and lagoonal deposits into marine carbonates.
•In the area between the Adige Valley and the Karawanken Mountains the Bellephron Formation ranges in thickness from 0 ( westernmost area ) to 360 m.
•Val Gardena Sandstone in the Bletterbach section was regarded as Uppermost Capitanian-Dzhulfian in age, while the Bellerophron formation was attributed to the Dorashamian-Changxingian.
•Continental sabkha facies association :The characters suggest a sabkha mudflat environment. The deposits may represent muddy sheet flows extending beyond the reach of the alluvial fans and ponded on the adjecant mudflats.
•Mixed-load channels :The general characteristics of the channelized bodies find close analogues in point-bar deposits from modern singe-channel rivers with sinuous thalwegs. The bipartite character of the point-bar sequence, with heterolithic inclined stratification grading downdip into trough cross-bedded sandstone, indicates mixed-load sinuous rivers of strongly contrasting energy regimes and considerably fluctuating discharge.
•Sandstone layers interbedded with overbank mudstone probably represents flood related levee/splay deposits, emplaced as a result of the catastrophic overtopping and breaching of the channel banks.
•Terminal fan facies :Rivers probably underwent a definite decrease in depth downstream and a sharp fall-off in their average discharge in the lower part of their courses, due to a number of factors including:
•A) Loss of water by evapo-transpiration and lower precipitation.
•B) Infiltration into a permeable alluvium.
•The rivers were probably exotic, and reduced to a network of low-gradient, shallow, ephemeral wadi distributaries plugged during repetive flash-flooding events and subject to frequent.
•Mouth bar :
•1) Coastal belt characterized by sabkha-type sediments interfingering with terminal fan deposits.
•2) Hypersaline lagoon or restricted inner shelf, characterized by evaporates and carbonate deposits.
•3) An inner carbonate shelf.
•The transition from coastal to offshore environments was extremely gradual, suggesting a very gentle depositional gradient, and resulting in sedimentation particularly sensitive to environmental changes. The Bellephron Formation is sharply or transitional overlain by a regional widespread member between different sections, in the absence of outcrop continuity.
• The Upper Permian succession of the Southern Alps displays the typical features of early rift successions. The overall pattern is that of transgressive, backstepping and onlapping arrangements of deposits suggesting a background of regional subsidence.
•Sedimentation started in graben-like depressions, and was initially fed by local sources. Later as a result of regional subsidence and morphogenetic evolution of the basin, the original depositional areas joined, ultimately resulting in an unified and increasingly expanded depression with sedimentary fill progressively onlapping the margins of the basin. Basin opening is recorded by coarse scree and alluvial fan deposits.
•The thickness of Val Gardena Sandstone in the study area ranges from zero ( Trento area ) to more than 500 m ( Comelico area ). In the Dolomites and Carnia, the red beds grade into the evaporitic and carbonate deposits of the Bellerophon Formation. The transgression progressively encroached on western areas. A structural high between the Adige Valley and the Giudicarie Line prevented the westward progression of the transgression, so that the Bellerophron Formation appears on a broad scale as a sedimentary wedge pinching out westwards. The Bellephron Formation is overlain by the Lombardian Unit ( continental sediment ).
•Alluvial fan facies Val Gardena Sandstone:Very poorly sorted and structureless sandy-gravelly red brown mudstone. Shows diamitic like textures, in which a variable amounts of granules and small pebbles, and rarely sparse sub angular to sub rounded pebbles or cobbles, float in a matrix of very poorly sorted mixture of sand, silt and clay. Bed thickness averages 100 cm and ranges 25-210 cm. Internal bedding and clast imbrication are lacking. Normal grading sometimes occurs in the coarser-grained varieties. Individual beds usually have flat, non-erosional basal contacts. Beds are locally capped by fine clast-supported conglomerates, either one clast thick or displaying a crude planar lamination in the finer grained varieties.
•Continental sabkha facies association :Consists of generally massive ( rarely finely laminated ) and sometimes heavily bioturbated red-brown to purple silty mudstone, locally with sparse sandy interbeds, mud cracks (either simple desiccation cracks or deep cracks reflecting the shrink-well behaviour of a vertic-type soil ) and layers of calcrete nodules.
•Mixed-load channels :The bedload channels grade upward into a complex of single-storey and or stacked multilateral channelized bodies showing tabular geometry, associated with variable amounts of overbank fines. Individual channel bodies range in thickness from 2 to 10 m, and usually consist of a heterolithic set of low angle sand/mud accretionary beds (epsilon cross-bedding or inclined heterolithic stratification) grading downdip into a sandy unit which generally lacks evidence of lateral accretion and consists of medium-to large-scale trough cross-bedded and locally planar laminated sandstone. A more or less regularly fining and thinning upward trend may be recognized in the channel bodies, in both grain sizes and scale of structures, although coarsening upwards sequences, coarsening to fining upwards, and unsystematic trends also occur. The sediments involved in the trough cross-bedded lower unit range in grain size from coarse sandstone and locally granule- or fine pebble conglomerates, to medium sandstone, whereas the overlying inclined heterolithic stratification (I.H.S.) set is generally finer-grained. Relatively thin channel bodies transitional to the terminal fan facies association may entirely fall in the very fine sand to silt size ranges; these obvious lack a trough cross-bedded unit, for a granulometric reasons, and the I.H.S. set extends into the base of the channel.
•Most channel bodies have low-relief erosional bases, and some of the better-exposed examples also display erosional cutbanks. The base is commonly overlain by a patchily development coarse lag of intraformational ( mudstone and or calcrete ) clasts, locally associated with extraformational pebbles and coalified plant fragments. Trough cross-bedded sandstones in the lower part of the channel commonly contain variable amounts in cosets, usually showing upward decrease in set height.
•The inclined layers of I.H.S. in dip section show an overall profile ranging from straight to slightly sigmoidal. Within some channel bodies, or horizontal segments of them, the portion exhibiting I.H.S. may develop over the whole thickness of the body, extending the downdip to the channel base, even in the presence of lithologies coarser than fine sand; more commonly, it encompasses a variable fraction of the whole thickness of the body. Larger outcrops show that the lower trough cross-bedded unit may develop over variable thicknesses within the same channel sandstone, locally thickening at the expense of the overlying I.H.S unit. The inclined heterolithic strata dip at angles ranging from 7° to 13°, essentially depending on the grainsize of sediments, with steeper angles observed in finer-grained I.H.S. sets.
•Individual I.H.S. units typically consist of a sharp-based sand mud couplet. The thicknesses of both the coarse and fine members of these couplets typically range from a few cm up to a few dm. The base of the sand member is either a depositional bedding plane, or an irregular broadly erosional surface; the contact with the overlying muddy member is usually sharp, less commonly gradational. Within individual inclined sand-mud couplets, fining commonly occurs both perpendicular to inclined bounding surface and updip. Within sandy members a sequence of structures indicating waning flow may sometimes be observed and lower flow regime structures tend to predominate in the uppermost part of the I.H.S. set. Fine members commonly appear structureless.
•Shallow channel with sandy and or muddy fill locally cut into the top of the channelized bodies and are occasionally accompanied by individual high-angle avalanche sets of planar cross-bedded sandstone at their downstream termination.
•Channel abandonment is marked by locally observed broadly lenticular fine-grained channel fills.
•Intervals of fine-grained sediments between single and amalgatamed sandstone bodies consist of red structureless, commonly pedogenically modified mudstone, locally interbedded with broadly lenticular tabular, dm to cm sandy layers, occasionally showing planar lamination to ripple laminated waning-flow structures. A low-angle wedge-shaped thin-interbedded bundle of alternating fine sand and mud layers sometimes occurs at the top of channelized bodies.
•Inclined sand/mud interbedding in the I.H.S. units of some point-bar sequences display thinner-bedded alternations and a remarkably higher degree of rhythmicy and regularity; in addition poor evidence of reversing flow may be locally observed and bioturbation, although characterized by low diversity, is more intense.
•Terminal fan facies :Generally single-storey ribbons and sheet like composite channelized bodies, generally of mixed load type, showing on average an upward increasing reduction in thickness and river bedload content; these are encased in an increasingly overbank of sediments, leading to low channel interconnections. This evolution is typically absent or very poorly developed in the marginal parts of the basin and in the less subsiding areas characterized by reduced thickness of sedimentary succession. Simple single-storey ribbons range from 0.9 to 4.3 m in thickness. The sediments involved range coarse sands, locally bearing granules and small pebbles, to fine sands and mudstone. Channel fills exhibit a sharp, fairly incised, irregular erosional base, sometimes with a remarkedly stepped appearance, floored by mudstone clasts and sometimes-reworked calcrete nodules. Vertically accreted ribbons commonly display a crudely developed fining-upward textural sequence, with trough cross bedding of upward decreasing scale, locally accompanied by planar lamination; small ribbons of this type may have been deposited by single flood events. Other ribbons show an inclined heterolithic stratification suggesting limited lateral migration, with muddy members of the couplets usually reaching the base of the channel.
•Thick muddy intercalations, sometimes pedogenically modified, may occur in the lateral accretionary units indicating that the fill was highly episodic in some channels. Burrow networks are relatively common in the upper part of the channel fills.
•Internal scouring surfaces, in place floored by mudstone clasts, are quite common, and several stages of scour and fill are characteristic, especially in channelized bodies of the inner more proximal zone of the system, and suggests highly fluctuating hydrodynamic conditions.
•Sheet like sandstones, mostsly resulting from lateral and to a limited extend also vertical amalgation of ribbons and small-storey internal organization can be found.
•Channel fills are interbedded with a great variety of interchannel sand and silt units deposited during flooding. Splay sandstones and commonly form a volumetrical large proportion of the total sand framework, especially near the transition into inferred sabkha sediments and may extend far into the interchannel areas, reflecting the extremely flashy and ephemeral nature of the associated streams. Splay deposits include both sheet sand layers showing virtual non-erosional basal contacts with the underlying sediments, and more discontinous and lenticular beds with broadly erosional bases. Sheet sandstone layers partly appear to be wings tapering away from channel sandstone bodies and sometimes coalescing to form extensive sheet-splays. The may be rich in vegetal debris and show small-scale flood sequences. Wave ripples draped by mud laminae are also locally observed.
•The upper surface of some play sandstone layers show evidence of a local shallow scouring. Splay sandstones and associated mudstones in places show root traces and or invertebrate burrows and are occasionally interbedded with dolomite and gypsum layers. Horizons with mud cracks in the mudstones.
•Spaly sandstones may appear organized into thickening and coarsening-upward sequences 6-10 m thick, displaying a clear upward increase in the sand/mud ratio. These consists of basal mudstone intervals with sparse, thin, sandstone interbeds passing up into thicker-bedded sheet-sandstones and eventually single channel fills.
•Mouth bar :Small-scale upward thickening and coarsening sequences 3-6 m thick are generally observed at the transition from lagoonal/marine to continental facies associations. Muddy layers alternate with thin-bedded sandstones in the lower part, and progressively disseappear, to grade upwards into a amalgated package of sandstone layers generally displaying unidirectional structures such as ripple-drift cross-lamination, trough cross bedding and planar-lamination; the sequence may be capped by erosive based channel sandstones. The facies association is thought to represents small mouth-bars encroaching on a shallow-water basin.
Size class: microfossils
Preservation: cast, mold/impression, adpression, original carbon, original sporopollenin, original cellulose
Collection methods: core, chemical, mechanical,
Primary reference: F. Massari, C. Neri, P. Pittau, D. Fontana, and C. Stefani. 1994. Sedimentology, palynostratigraphy and sequence stratigraphy of a continental to shallow-marine rift-related succession: Upper Permian of the eastern Southern Alps (Italy). Mem. Sci. Geol. 46:119-234 [C. Looy/W. Puijk/C. Looy]more details
Purpose of describing collection: paleoecologic analysis
PaleoDB collection 32197: authorized by Cindy Looy, entered by Wilma Puijk on 10.06.2003
Creative Commons license: CC BY (attribution)