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Facies analysis of a Pliocene river-dominated deltaic succession (Siena Basin, Italy): Implications for the formation and infilling of terminal distributary channels
Full text linkThe recent recognition and description of terminal distributary channels in ancient delta-front deposits have led to important progress when it comes to understanding: delta construction processes; the plan-view shapes of both modern and ancient river-dominated deltas; and sediment dispersal within deltas. Deltaic deposits fed by terminal distributary channels can be observed in the extensive outcrops (approximately up to 300 m wide and 50 to 70 m thick) in the central sector of the Pliocene Siena Basin in Tuscany, Italy. Stratigraphic constraints indicate that the deposition occurred in relatively shallow marine settings, was not affected by tidal effects and was influenced only locally by wave action. As a consequence, these rock exposures provide an opportunity to investigate the facies assemblage of an ancient delta complex, which was built up almost exclusively as the result of river-related processes. The studied deltaic succession is composed of vertically stacked parasequence-like units, which are invariably characterized by the lack of coarse-grained distributary channel deposits at the top and an extensive network of terminal distributary channels that controlled the deltaic deposition. These deposits were interpreted previously as deep-water turbidites. This study provides new elements for the correct distinction between hyperpycnal-dominated delta fronts and basinal turbidite deposits. The terminal distributary channel facies documented in this study are more complex than those investigated in previous research. The channels are often passively filled during abandonment phases, and not just due to the aggradation and lateral/upstream migration of mouth bars, as classically proposed. Moreover, the frequent occurrence of mudstone beds just above the basal channel scours may be indicative of an abandonment phase after the channel incision, and before its infilling, with subsequent implications for the formational and infilling mechanisms of terminal distributary channels. Furthermore, the channel fills are often the result of alternating depositional and erosional phases, leading to a complex stratigraphic arrangement
Shoal-water deltas in high-accommodation settings: Insights from the lacustrine Valimi Formation (Gulf of Corinth, Greece)
Full text linkThe stratigraphic architecture of shoal-water deltaic systems developed in low-accommodation settings is relatively well-known. In contrast, the features of shoal-water deltas developed in high-accommodation settings remain relatively poorly documented, especially when compared with the available data sets for Gilbert-type deltaic systems developed in the same settings. The lacustrine Valimi Formation (Gulf of Corinth, Greece) provides an opportunity to investigate the facies assemblage and architectural style of shoal-water deltaic systems developed in high-accommodation settings. The studied interval accumulated during the Pliocene and Pleistocene and represents part of the early syn-rift Gulf of Corinth succession. Six facies associations, each described in terms of depositional processes and geometries, have been identified and interpreted to represent a range of proximal to distal deltaic sub-environments: delta plain, distributary channel, mouth-bar, delta front, prodelta and open lake. The facies associations and their architectural elements reveal characteristics which are not common in traditionally described shoal-water deltas. Of note, different facies arrangements are observed in the distributary channels in different sectors of the delta, passing from thick single-storey channel fills embedded within delta-plain fines in landward positions, to thin, amalgamated and multi-storey channels closer to the river mouth. This study proposes a new depositional model for shoal-water deltas in high-accommodation settings documenting, for the first time, that shoal-water delta deposits can form a substantial part of stratigraphic successions that accumulate in these settings. The proposed depositional model provides new criteria for the recognition and interpretation of these deposits; the results of this study have applied significance for reservoir characterization
Depositional and palaeoecological features of the Middle Pliocene (Piacenzian) marine carbonates exposed in the Valdelsa Basin (San Gimignano, Siena - Italy)
No full textThe metamorphic units of the eastern side of Monte Leoni (Northern Apennines, Italy)
No full textThis work deals with the stratigraphic and structural setting of the metamorphic succession exposed in the eastern side of the Monte Leoni, the southern extension of the Monticiano-Roccastrada Ridge. In the study area, three main tectonic units crop out, which are, from top to bottom: the Ophiolitic Unit, the Tuscan Nappe and the epimetamorphic Monticiano-Roccastrada Unit. They are unconformably overlain by post-orogenic, Neogene marine and Quaternary continental deposits. The Monticiano-Roccastrada Unit consists of Palaeozoic deposits overlain by the Triassic Verrucano Group, represented by the Civitella Marittima Fm. (Early?-Middle? Triassic), the Anageniti minute Fm. (Late Ladinian) and the Tocchi Fm. (Carnian). The Verrucano Group of the eastern side of Monte Leoni lacks the Monte Quoio Fm., which is extensively exposed in the northern part of the Monticiano-Roccastrada Ridge and pinches out southward being progressively replaced by the Anageniti minute Formation.
The Palaeozoic deposits comprise the Falsacqua Fm., mainly consisting of normally graded metasandstones and metapelites, locally overlain conformably by a decametre-thick horizon of metacarbonates alternating with subordinate metapelites. The whole Palaeozoic succession exposed in the eastern side of Monte Leoni shows stratigraphic and sedimentological features which typically characterise the Farma Fm., considered in the type-area (Farma Valley, in the central part of the Monticiano-Roccastrada Ridge) as a Moscovian turbiditic deposits. As a consequence these pre-Verrucano deposits represent the southernmost outcrops of the Farma Formation. The Civitella Marittima Fm. unconformably overlies the Palaeozoic deposits and is represented by three members, which are: the basal mainly conglomeratic member, the intermediate mainly arenaceous member and the upper pelitic-arenaceous member. The Civitella Marittima Fm. records a retrogradational fluvial fan setting, possibly represented by coalescing terminal fans. The Anageniti minute Fm. erosionally overlies the fine-grained siliciclastics of the upper member of Civitella Marittima Fm. and consists of finingupwards successions represented by quartzose metaconglomerates, quartzose metasandstones and metapelites. These record ephemeral stream and flood-plain sedimentation, whereas the carbonates and siliciclastic fines of the overlying Tocchi Fm. documents the Carnian marine transgression over such an alluvial plain. In the Monte Leoni the Verrucano Group is formed by two sedimentary cycles. The lower one, stratigraphically corresponding to the lower sedimentary cycle of the Punta Bianca area (La Spezia), is represented by the Civitella Marittima Fm., whereas the upper cycle, comprising the Anageniti minute Fm. and the Tocchi Fm., is referable to the Verrucano succession (Verruca Fm. and Monte Serra Fm.) of the Monti Pisani. Four deformational events (D1, D2, D3 and D4) related to the Northern Apennines tectonic evolution affected the metamorphic units. D1 is represented by meso- to map-scale south- and east-verging F1 folds, with associated penetrative S1 axial planar tectonic foliation. S1 consists of a pervasive schistosity, particularly well developed in the metapelites, ranging from parallel to strongly inclined with respect to the bedding (S0). D2 is represented by asymmetric east-verging folds (F2), characterised by overturned to vertical shorter limbs. The F2 are typified by NE-SW striking axes and subhorizontal to gently NW/SE dipping axial surfaces. A penetrative, axial-planar tectonic foliation (S2) occurs in the metapelites. D3 is represented by map-scale gentle folds (F3) and by disjunctive and widely spaced S3 tectonic foliation. Finally, high-angle normal faults (D4) dissect all the previous structures
The role of sediment supply in large-scale stratigraphic architecture of ancient Gilbert-type deltas (Pliocene Siena-Radicofani Basin, Italy)
Full text linkAggradation, progradation and retrogradation are the main patterns that define the large-scale architecture of Gilbert-type deltas. These patterns are governed by the ratio between the variation in accommodation space and sediment supply experienced during delta growth. Sediment supply variations are difficult to estimate in ancient settings; hence, it is rarely possible to assess its significance in the large-scale stratigraphic architecture of Gilbert-type deltas. This paper presents a stratigraphic analysis of a Pliocene deltaic complex composed of two coeval and narrowly spaced deltaic branches. The two branches recorded the same tectonic- and climate induced accommodation space variations. As a result, this deltaic complex represents a natural laboratory for testing the effects of sediment supply variations on the stratigraphic architecture of Gilbert-type deltas. The field data suggest that a sediment supply which is able to counteract the accommodation generated over time promotes the aggradational/progradational attitude of Gilbert-type deltas, as well as the development of thick foreset deposits. By contrast, if the sediment supply is not sufficient for counterbalancing the generated accommodation, an aggradational/retrogradational stratigraphic architecture is promoted. In this case, the deltaic system is forced to withdraw during the different phases of generation of accommodation, with the subsequent flooding of previously deposited sub-horizontal topset deposits (i.e., the delta plain). The subsequent deltaic progradation occurs above these deposits and, consequently, the available space for foresets growth is limited to the water depth between the base-level and the older delta plain.This leads to the vertical stacking of relatively thin deltaic deposits with an overall aggradatational/retrogradational attitude
L'Isola di Pianosa: perla neogenica dell'Arcipelago Toscano
No full textPianosa è l'unica fra le sette isole dell'Arcipelago Toscano, tutte bellissime,
ad essere formata esclusivamente da rocce
sedimentarie di età neogenica. Le altre sono costituite
diversamente: Gorgona da rocce metamorfiche,
Capraia da rocce vulcaniche effusive, Giglio e
Montecristo da rocce magmatiche intrusi ve, Giannutri
da rocce calcaree triassiche, mentre l'Elba è
formata da rocce di genesi ed età assai diverse.
L'ubicazione geografica, approssimativamente
equidistante' fra la costa toscana e la costa corsa
(Piana di Aleria) con i rispettivi depositi neogenici,
fanno di Pianosa un elemento chiave nella ricostruzione
delle tappe evolutive dell ' apertura del
Mare Tirreno Settentrionale
Carta geologica dell'area Albola-Cintoia(Chianti Settentrionale Prov. di Firenze, Siena e Arezzo)
No full textCarta geologica del Chianti Settentrional
Chronostratigraphic revision of some Pliocene basins in Tuscany at the Zanclean/Piacenzian boundary
No full textThe data on Tuscan Pliocene basins (which were acquired during recent years) are revised and re-evaluated here in the light of the formal subdivision into three stages (Zanclean, Piacenzian and Gelasian) of the Pliocene Series. Particularly, data on the sediments close to the Zanclean/Piacenzian boundary are considered. These sediments registered important tectonic changes which can be accurately placed both chronostratigraphically and chronologically.
The Pliocene sedimentation in Tuscany started at the Messinian/Zanclean boundary (5.33 Ma) and ended in the uppermost Piacenzian (about 2.4 Ma). Some areas were characterised by a continuous sedimentation, while in other areas two depositional cycles were realised. in the second case the youngest sediments of the first cycle are refer-red to the Reticulofenestra pseudoumbilica LO (3.89 Ma)Globorotalia puncticulata LCO (3.59 Ma) interval, and the basal levels of the second cycle closely postdate the Globorotalia crassaformis gr. FCO (3,1 Ma). Therefore, a sedimentary hiatus of about 500 ky separates the two cycles. The end of the first cycle and the beginning of the second were caused by events which also determined sedimentary and environmental changes in the continuous succession
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