Miocene Marnoso-arenacea Formation Research Articles

Spatial and spacing distribution of joints at (over-) saturation in the turbidite sandstones of the Marnoso-Arenacea Fm. (Northern Apennines, Italy)

Despite that many field studies on joint attribute statistics are available in the literature, there is still an ongoing debate on the most effective statistical treatment to properly describe the distribution of a fundamental parameter like joint spacing. This may also relate to the objective difficulty of collecting very large datasets from single linear scanlines directly in the field, because of limited exposures. To overcome such a limitation, we selected a spectacular outcrop of the Miocene Marnoso-Arenacea Formation turbidite sandstones located near the Coniale village, in the Northern Apennines (Italy), where a vertical cliff provides a 100% continuous outcrop about 90 m high and 280 m long, with well-accessible exposures at its base, along the Santerno River. This setting allows combining data acquisition on a photogrammetric DOM (digital outcrop model) covering a representative sector of the outcrop, with ground truthing by direct measurements in the field. More than 2000 joints were identified in 9 linear scanlines traced in the DOM along mechanical layers with different thickness. Comparison with field data indicates that both joint orientation and spacing data from the DOM are very reliable. The scanlines include between 549 and 37 joints, depending on layer thickness, allowing to perform robust statistical analysis. The Kolmogorov-Smirnov (K–S), Cramer-von Mises (CvM) and Anderson-Darling (A-D) nonparametric tests were used to identify the probability density function(s) that best fit spacing data among exponential, Gamma, log-normal and normal distributions. The Gamma distribution turned out as the most probable in most cases. In some specific sectors along all scanlines, joint density is significantly greater than the average for the presence of joint clusters. In some cases, joint clusters are vertically aligned with those in underlying and/or overlying scanlines, thus resulting in thoroughgoing joint corridors. Comparison of spacing statistics inside and outside joint corridors/clusters indicates a typical transition from Gamma distributions outside joint corridors/clusters, to log-normal distributions inside them. The presence of joint clusters and corridors, as well as FSR, mode/mean ratio, JSR, and FSI values, suggests that diffuse jointing in the sandstone layers of the turbiditic succession reached the saturation stage before a normal probability density function was produced, and then further strain was accommodated by localized deformation in clusters and corridors.

Application of the adjoint approach to optimise the initial conditions of a turbidity current with the AdjointTurbidity 1.0 model

Abstract. Turbidity currents are one of the main drivers of sediment transport from the continental shelf to the deep ocean. The resulting sediment deposits can reach hundreds of kilometres into the ocean. Computer models that simulate turbidity currents and the resulting sediment deposit can help us to understand their general behaviour. However, in order to recreate real-world scenarios, the challenge is to find the turbidity current parameters that reproduce the observations of sediment deposits. This paper demonstrates a solution to the inverse sediment transportation problem: for a known sedimentary deposit, the developed model reconstructs details about the turbidity current that produced the deposit. The reconstruction is constrained here by a shallow water sediment-laden density current model, which is discretised by the finite-element method and an adaptive time-stepping scheme. The model is differentiated using the adjoint approach, and an efficient gradient-based optimisation method is applied to identify the turbidity parameters which minimise the misfit between the modelled and the observed field sediment deposits. The capabilities of this approach are demonstrated using measurements taken in the Miocene Marnoso-arenacea Formation (Italy). We find that whilst the model cannot match the deposit exactly due to limitations in the physical processes simulated, it provides valuable insights into the depositional processes and represents a significant advance in our toolset for interpreting turbidity current deposits.

Sedimentary facies control on mechanical and fracture stratigraphy in turbidites

Natural fracture networks exert a first-order control on the exploitation of resources such as aquifers, hydrocarbons, and geothermal reservoirs, and on environmental issues like underground gas storage and waste disposal. Fractures and the mechanical stratigraphy of layered sequences have been intensively studied to unravel the relationships between bed thickness and fracture spacing, but less attention has been paid to intrabed fracturing patterns due to the intrinsic local variability of sedimentary processes and products. Among sedimentary rocks, turbidites show great lateral and vertical variability of textural characteristics and depositional facies, which are expected to strongly influence the location and density of fractures. To better understand the contribution of stratigraphic, sedimentologic, and petrophysical properties on fracture patterns, we performed a high-resolution study on a selected stratigraphic interval of jointed foredeep turbidites in the Miocene Marnoso-Arenacea Formation (Northern Apennines, Italy). Cumulative statistical relations of field and laboratory structural, sedimentologic, and petrophysical data significantly improved when analyzed at the sedimentary facies scale. In particular, for facies recording different cross-flow (i.e., longitudinal to the paleocurrents) depositional conditions within the parent turbidity currents, we observed three-dimensional anisotropies of rock hardness (i.e., uniaxial compression) that were positively correlated with normalized fracture intensities, indicating a primary sedimentary control on fracture distribution. This type of intrabed joint distribution has crucial practical implications for the lateral prediction and evaluation of mesoscale fracture patterns in turbidite sequences.

Determining flow directions in turbidites: An integrated sedimentological and magnetic fabric study of the Miocene Marnoso Arenacea Formation (northern Apennines, Italy)

Depositional models of turbidity flows require estimating paleocurrent directions using sedimentological indicators such as flute and ripple marks, but these are not always present in outcrop sections or drill cores. In this study, we apply the anisotropy of magnetic susceptibility (AMS) as an alternative tool to estimate paleocurrent directions in a case-study turbiditic system: the Marnoso Arenacea foredeep turbidites of Miocene age exposed in the northern Apennines of Italy. Different depositional facies have been sampled for AMS and additional rock-magnetic analyses. We observed a good agreement between paleocurrent directions from flute casts at the base of turbidite beds and mean directions of maximum magnetic susceptibility axes in organized facies such as massive and laminated sandstones, even if a relatively small but apparently consistent offset of ~15–20° seems to be present. Highly dispersed AMS fabrics were instead observed in disordered facies such as convoluted and undulated sandstones as well as debrites. This strong correlation between hydrodynamic regimes of depositional facies and AMS data represents a novel contribution and confirms the validity of the AMS method to estimate flow directions in the absence of sedimentological indicators. Finally, paleomagnetic analyses from the literature were used to reconstruct the paleogeography of the Marnoso Arenacea basin and make inferences about the origin and direction of transport of the sediments at the basin scale.

Quantitative Analysis of Submarine-Flow Deposit Shape In the Marnoso-Arenacea Formation: What Is the Signature of Hindered Settling From Dense Near-Bed Layers?

Abstract Submarine sediment density flows are one of the volumetrically most important processes for sediment transport across Earth. The sediment concentration of flows that reach the deep ocean has never been measured directly, and understanding these long-runout flows remains a major challenge. The Miocene Marnoso-Arenacea Formation in the Italian Apennines is the only ancient sequence where individual submarine sediment-density-flow deposits (single beds) have been mapped out for more than 100 km down-flow. Here we document the external shape and internal architecture of thirty-two individual beds that record flow evolution and can be compared to deposit shapes in mathematical or experimental models. The large number of beds allows modes of flow behavior to be identified. Larger-volume turbidites are typically dominated by massive (TA) or planar-laminated (TB) sandstone intervals that have a broad thickness maximum. This shape is important because it suggests that massive and planar laminated sandstones record hindered settling from dense near-bed layers, which have high (> 10% by volume) sediment concentrations. Previously, some authors have inferred that planar-laminated sandstones (TB) are deposited mainly by dilute flows. The position of the broad thickness maximum moves basinward as the volume of sand in the flow increases. This is consistent with mathematical modeling that suggests the position of the thickness maximum depends on flow thickness, flow speed, and sediment settling velocity, as well as sediment concentration, variations in seawater entrainment rate, and local changes in seafloor gradient. Smaller-volume turbidite sandstone intervals are finer grained and dominated by ripple cross-lamination (TC) and have a near exponential decay in thickness that is consistent with deposition from a dilute sediment suspension. The rate of near exponential thinning is controlled by sandstone volume. In contrast, turbidite mudstone intervals show an approximately linear increase in thickness with distance. Flows that entered the basin in opposite directions produced turbidite mudstone intervals that thicken towards the same location, indicating that muddy turbidity currents can drain back over long distances to basinal lows.

Space/time tectono-sedimentary evolution of the Umbria-Romagna-Marche Miocene Basin (Northern Apennines, Italy): a foredeep model

The space/time evolution of the Umbria-Romagna-Marche domains of the northern Apennine Miocene foredeep is proposed. In this period, the turbidite siliciclastic sedimentation is represented mainly by the Miocene Marnoso-Arenacea Formation, which generally ends with mainly marly deposits. From the internal Apennine sectors (Umbria-Romagna domain) to the external Adriatic Margin (Marche domain) the siliciclastic succession overlies hemipelagic marly deposits (Schlier Formation). The whole depositional area can be considered as a single wide basin with depocenter or main sedimentation areas progressively migrating eastwards. This basin is characterized by some morphological highs which did not constitute real dams for the sedimentary flows (turbidity currents). Multiple feeding (arkose, litharenites, calcarenites) from different sources is related to palaeogeographical and palaeotectonic reorganization of the most internal, previously deformed, Apennine areas. The activation of the foredeep stage is marked by the beginning of the siliciclastic sedimentation (Late Burdigalian in the most internal sector). This sedimentation ends in the most external sector in the Early Messinian, pointing to a depositional cycle of about 9–10 Ma. The diachronism of the base of the siliciclastic deposition proves to be almost 5 Ma. The syn-depositional compressional deformation, which shows a marked diachronism, affected the internal area of the foredeep in the Early-Middle Serravallian, and progressively migrated up to Late Miocene, involving more and more external sectors. The deformed siliciclastic sedimentary wedge constitutes an orogenic pile incorporated in the Apennine Chain, represented by different tectonic elements superimposed by means of NE-vergent thrusts. The main stratigraphic and tectonic events of the Toscana-Romagna-Marche Apennines are presented in a general framework, resulting also in a terminological revision.

Facies architecture of individual basin‐plain turbidites: Comparison with existing models and implications for flow processes

AbstractCurrent understanding of submarine sediment density flows is based heavily on their deposits, because such flows are notoriously difficult to monitor directly. However, it is rarely possible to trace the facies architecture of individual deposits over significant distances. Instead, bed‐scale facies models that infer the architecture of ‘typical’ deposits encapsulate current understanding of depositional processes and flow evolution. In this study, the distribution of facies in 12 individual beds has been documented along downstream transects over distances in excess of 100 km. These deposits were emplaced in relatively flat basin‐plain settings in the Miocene Marnoso Arenacea Formation, north‐east Italy and the late Quaternary Agadir Basin, offshore Morocco. Statistical analysis shows that the most common series of vertical facies transitions broadly resembles established facies models. However, mapping of individual beds shows that they commonly deviate from generalized models in several important ways that include: (i) the abundance of parallel laminated sand, suggesting deposition of this facies from both high‐density and low‐density turbidity current; (ii) three distinctly different types of grain‐size break, suggesting waxing flow, erosional hiatuses and bypass of silty sediment; (iii) the presence of mud‐rich debrites demonstrating hybrid flow deposition; and (iv) dune‐scale cross‐lamination in fine‐medium grained sandstones. Submarine sediment density flows in basin‐plain settings flow over relatively simple topography. Yet, their deposits record complex flow events, involving transformation between different flow types, rather than the simple waning surges often associated with the distal parts of turbidite systems.

Planform geometry, stacking pattern, and extrabasinal origin of low strength and intermediate strength cohesive debris flow deposits in the Marnoso-arenacea Formation, Italy

The Miocene Marnoso-arenacea Formation (Italy) is the only ancient sequence where deposits of individual submarine density flow deposits have been mapped in detail for long (>100 km) distances, thereby providing unique information on how such flows evolve. These beds were deposited by large and infrequent flows in a low-relief basin plain. An almost complete lack of bed amalgamation aids bed correlation, and resembles some modern abyssal plains, but contrasts with ubiquitous bed amalgamation seen in fan-lobe deposits worldwide. Despite the subdued topography of this basin plain, the beds have a complicated character. Previous work showed that a single flow can commonly comprise both turbidity current and cohesive mud-rich debris flows. The debris flows were highly mobile on low gradients, but their deposits are absent in outcrops nearest to source. Similar hybrid beds have been documented in numerous distal fan deposits worldwide, and they represent an important process for delivering sediment into the deep ocean. It is therefore important to understand their origin and flow dynamics. To account for the absence of debrites in proximal Marnoso-arenacea Formation outcrops, it was proposed that debris flows originated within the study area due to erosion of mud-rich seafloor; we show that this is incorrect. Clast and matrix composition show that sediment within the cohesive debris flows originated outside the study area.

Turbidites in the Miocene Marnoso Arenacea Formation, northern Apennines, Italy

Turbidites in the Miocene Marnoso Arenacea Formation, northern Apennines, Italy

Synsedimentary structural control on foredeep turbidites: An example from Miocene Marnoso-arenacea Formation, Northern Apennines, Italy

This work discusses the synsedimentary structural control affecting the turbidites of the Marnoso-arenacea Formation (MAF) deposited in an elongate, NW-stretched foredeep basin formed in front of the growing Northern Apennines orogenic wedge. The stratigraphic succession of the MAF (about 4000 m thick) records the progressive closure of the Apennine foredeep basin due to the NE propagation of thrust fronts. In this setting, Langhian to Serravallian turbidites are overlain by Tortonian mixed turbidite deposits, i.e. sandstone-rich low-efficiency turbidites. The high-resolution stratigraphic framework of basin-plain turbidites has made it possible to identify five informal stratigraphic units (I, II, III, IV, V) mainly on the basis of the structural control highlighted by: 1) the presence of topographic highs and relative depocentres detected through a progressive flattening approach, and 2) the presence of thrust-related mass-transport complexes and the progressive appearance and disappearance of five bed types (Types 1, 2, 3, 4, 5) considered important to understand the interaction between flow efficiency and basin morphology. By contrast, the upper part of the MAF succession (Tortonian in age) is formed by more sandstone-rich systems characterized by beds whose origin is likely to depend, at least in part, upon flow decelerations related to topographic confinement due to the progressive closure of the foredeep. The vertical and lateral distribution of these types of beds is, therefore, useful for the reconstruction of the morphological evolution of structurally controlled basins; in the MAF example, this is mainly due to the progressive narrowing of the foredeep caused by the propagation of the main thrust fronts toward the foreland.

Sedimentary filling of a wedge-top basin and relationship with the foredeep (Middle Miocene Marnoso-arenacea Formation, northern Apennines)

The Middle Miocene Marnoso-arenacea Formation at Deruta in the northern Apennines of Italy rests unconformably on an orogenic wedge adjacent to the Adriatic foredeep. Based on a detailed facies analysis, the succession reveals two genetically related depositional systems: a distal delta-fed sand-rich system and a more proximal fan-delta slope system. Petrographic data confirm the genetic relationship between the two depositional systems, with the fan-delta slope feeding the basinward sand-rich system. The Deruta depositional setting shows a multi-step sedimentary evolution controlled by tectonically induced relative sea-level changes. The first stage, corresponding to a sea-level rise, promoted deposition in a wedge-top basin of pebbly sand and sand lobes (delta-fed). The second stage, characterized by intense tectonic activity (uplift) and sea-level fall, promoted accumulation of a prograding fan-delta slope replacing the sand-rich lobes. This phase was dominated by mass failures and methanogenic cold seepages. During these two stages, the wedge-top basin was isolated from the adjacent foredeep. Only during the third stage was a connection established, with the development of a deep-sea fan in the foredeep, fed by a deltaic depositional system.

Evolution of Turbidity Currents Deduced from Extensive Thin Turbidites: Marnoso Arenacea Formation (Miocene), Italian Apennines

Previous work has shown that large-volume (> 100 km3) turbidity currents are capable of depositing extensive (> 100 km) turbidites in non-channelized settings. This study of the Miocene Marnoso Arenacea Formation demonstrates that smaller-volume (< ~ 0.5 km3) turbidites can also be remarkably extensive. Some thin (< 40 cm) turbidites extend for 120 by 30 km within this non-channelized sequence. Flows that produced these thin turbidites were capable of traversing the basin plain in opposing directions, despite being too weak to cause significant substrate erosion. Some flows traveled upslope for at least 60 km. Long run-out distances result from the relatively large fraction of fine particles in these highly efficient flows. Sandstone intervals thin in an approximately exponential fashion, across the proximal 20 to 30 km of outcrop. Similar sandstone shapes are observed for flows that traversed the basin plain in opposite directions. This sandstone shape is a generic property of spatially depletive flows; it is consistently reproduced by laboratory and numerical models in which velocity, sediment concentration, and deposition rates decrease down-flow. Rates of sandstone thinning are orders of magnitude smaller than those observed in deposits from laboratory experiments. This discrepancy primarily reflects a low ratio of flow thickness to sediment settling velocity, and low rates of sediment suspension from the bed, for slower-moving laboratory flows. The distal part of the thin-bed sandstone interval is thinner (< 5 cm) and more tabular. Starved ripples suggest that this distal tabular sandstone shape results partly from bedload reworking. The thickest turbidite mudstone occurs in the same two locations, for flows that traveled across the basin in opposite directions. This suggests that substantial volumes of muddy fluid ponded within bathymetric lows. Abrupt lateral termination of two mudstone beds, which have a distinctly tabular shape, suggests that certain mudstone intervals were deposited by dense fluid mud layers rather than expanded turbulent suspensions.

Anatomy of turbidites and linked debrites based on long distance (120 × 30 km) bed correlation, Marnoso Arenacea Formation, Northern Apennines, Italy

AbstractMuch of our understanding of submarine sediment‐laden density flows that transport very large volumes (ca 1 to 100 km3) of sediment into the deep ocean comes from careful analysis of their deposits. Direct monitoring of these destructive and relatively inaccessible and infrequent flows is problematic. In order to understand how submarine sediment‐laden density flows evolve in space and time, lateral changes within individual flow deposits need to be documented. The geometry of beds and lithofacies intervals can be used to test existing depositional models and to assess the validity of experimental and numerical modelling of submarine flow events. This study of the Miocene Marnoso Arenacea Formation (Italy) provides the most extensive correlation of individual turbidity current and submarine debris flow deposits yet achieved in any ancient sequence. One hundred and nine sections were logged through a ca 30 m thick interval of time‐equivalent strata, between the Contessa Mega Bed and an overlying ‘columbine’ marker bed. Correlations extend for 120 km along the axis of the foreland basin, in a direction parallel to flow, and for 30 km across the foredeep outcrop. As a result of post‐depositional thrust faulting and shortening, this represents an across‐flow distance of over 60 km at the time of deposition. The correlation of beds containing thick (&gt; 40 cm) sandstone intervals are documented. Almost all thick beds extend across the entire outcrop area, most becoming thinly bedded (&lt; 40 cm) in distal sections. Palaeocurrent directions for flow deposits are sub‐parallel and indicate confinement by the lateral margins of the elongate foredeep. Flows were able to traverse the basin in opposing directions, suggesting a basin plain with a very low gradient. Small fractional changes in stratal thickness define several depocentres on either side of the Verghereto (high) area. The extensive bed continuity and limited evidence for flow defection suggest that intrabasinal bathymetric relief was subtle, substantially less than the thickness of flows. Thick beds contain two distinct types of sandstone. Ungraded mud‐rich sandstone intervals record evidence of en masse (debrite) deposition. Graded mud‐poor sandstone intervals are inferred to result from progressive grain‐by‐grain (turbidite) deposition. Clast‐rich muddy sandstone intervals pinch‐out abruptly in downflow and crossflow directions, in a fashion consistent with en masse (debrite) deposition. The tapered shape of mud‐poor sandstone intervals is consistent with an origin through progressive grain‐by‐grain (turbidite) deposition. Most correlated beds comprise both turbidite and debrite sandstone intervals. Intrabed transitions from exclusive turbidite sandstone, to turbidite sandstone overlain by debrite sandstone, are common in the downflow and crossflow directions. This spatial arrangement suggests either: (i) bypass of an initial debris flow past proximal sections, (ii) localized input of debris flows away from available sections, or (iii) generation of debris flows by transformation of turbidity currents on the basin plain because of seafloor erosion and/or abrupt flow deceleration. A single submarine flow event can comprise multiple flow phases and deposit a bed with complex lateral changes between mud‐rich and mud‐poor sandstone.

Chemosynthetic communities as fingerprints of submarine sliding-linked hydrocarbon seepage, Miocene deep-sea strata of the Tuscan–Romagna Apennines, Italy

A specialised low-diversity bivalve assemblage dominated by chemosymbiotic vesicomyid bivalves (?Calyptogena sp.) is associated with a large submarine landslide (Casaglia-M. della Colonna slide) within the turbiditic Miocene Marnoso-arenacea Formation in the northern Apennines. This macrofossiliferous horizon is located just at the top of the slide close to the boundary between intrabasinal deformed sediments and extrabasinal sediments. There is a causal relationship between the emplacement of this large submarine slide and fluid expulsion promoting the localised and ephemeral establishment of specialised communities. The occurrence of exclusively sulphur-based biota and the concomitant absence of 13C-depleted authigenic carbonates from the host sediment are likely an indication that hydrocarbons within subsurface fluids were consumed through sulphate reduction processes. As a result, high levels of shallow-seated H2S were produced and then exploited by chemosymbiotic bivalves.

Beds comprising debrite sandwiched within co‐genetic turbidite: origin and widespread occurrence in distal depositional environments

AbstractCo‐genetic debrite–turbidite beds occur in a variety of modern and ancient turbidite systems. Their basic character is distinctive. An ungraded muddy sandstone interval is encased within mud‐poor graded sandstone, siltstone and mudstone. The muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite. The mud‐poor sandstone, siltstone and mudstone show features indicating progressive layer‐by‐layer deposition and are thus called a turbidite. Palaeocurrent indicators, ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event. Detailed field observations are presented for co‐genetic debrite–turbidite beds in three widespread sequences of variable age: the Miocene Marnoso Arenacea Formation in the Italian Apennines; the Silurian Aberystwyth Grits in Wales; and Quaternary deposits of the Agadir Basin, offshore Morocco. Deposition of these sequences occurred in similar unchannellized basin‐plain settings. Co‐genetic debrite–turbidite beds were deposited from longitudinally segregated flow events, comprising both debris flow and forerunning turbidity current. It is most likely that the debris flow was generated by relatively shallow (few tens of centimetres) erosion of mud‐rich sea‐floor sediment. Changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition. The association with distal settings results from the ubiquitous presence of muddy deposits in such locations, which may be eroded and disaggregated to form a cohesive debris flow. Debrite intervals may be extensive (&gt; 26 × 10 km in the Marnoso Arenacea Formation) and are not restricted to basin margins. Such long debris flow run‐out on low‐gradient sea floor (&lt; 0·1°) may simply be due to low yield strength (≪ 50 Pa) of the debris–water mixture. This study emphasizes that multiple flow types, and transformations between flow types, can occur within the distal parts of submarine flow events.

Anatomy and emplacement mechanism of a large submarine slide within a Miocene foredeep in the northern Apennines, Italy: A field perspective

The Casaglia Monte della Colonna (CMC) body, from the middle Miocene Marnoso-arenacea Formation of the Northern Apennines, covering more than 350 square kilometers, provides a rare opportunity to study the geometry and the internal deformation of a large, basin-wide submarine landslide. Although the head zone of the landslide has not been preserved, the body has a transitional zone, a contractional ramp cutting off more than 200 meters of the footwall stratigraphic succession, and a very wide lobe spilling over onto the adjoining basin plain deposits. The extent of the body, the depth of the ramp cut-off and the widening of the lobe are comparable with the largest present-day submarine landslides, not often observed in fossil examples. Internal deformation structures are distributed differently in the ramp and lobe zones. The ramp zone has antiformal stacking of duplexes and steeply inclined folds. The lobe has a well-defined strain partition: the upper part of the body is affected by extensional structures such as listric normal faults and extensional duplexes; the lower part is deformed by recumbent folds, boudinage and stacking of blocks, which are compatible with flow-induced, heterogeneous simple shear. This distribution is consistent with a kinematic model of extrusion-spreading, implying rear compression in the ramp zone with thickening and shortening of the body, and spreading in the lobe with thinning and stretching. The localized buckling present in the distal part of the lobe may be related to the onset of lateral confinement, due to local topographic features of the basin plain (synsedimentary intrabasinal high). The various degrees and styles of stratal disruption in the mass wasting body depend on the different combination of progressive simple shear, layer-parallel extension and shortening. Different structural associations occur in well-defined parts of the body due to kinematics of emplacement (spreading, flow and buckling). These associations may be diagnostic of gravitational mass wasting processes rather than shallow-level tectonic deformation.

On the frequency distribution of turbidite thickness

The frequency distribution of turbidite thickness records information on flow hydrodynamics, initial sediment volumes and source migration and is an important component of petroleum reservoir models. However, the nature of this thickness distribution is currently uncertain, with log‐normal or negative‐exponential frequency distributions and power‐law cumulative frequency distributions having been proposed by different authors. A detailed analysis of the Miocene Marnoso Arenacea Formation of the Italian Apennines shows that turbidite bed thickness and sand‐interval thickness within each bed have a frequency distribution comprising the sum of a series of log‐normal frequency distributions. These strata were deposited predominantly in a basin‐plain setting, and bed amalgamation is relatively rare. Beds or sand intervals truncated by erosion were excluded from this analysis. Each log‐normal frequency distribution characterizes bed or sand‐interval thickness for a given basal grain‐size or basal Bouma division. Measurements from the Silurian Aberystwyth Grits in Wales, the Cretaceous Great Valley Sequence in California and the Permian Karoo Basin in South Africa show that this conclusion holds for sequences of disparate age and variable location. The median thickness of these log‐normal distributions is positively correlated with basal grain‐size. The power‐law exponent relating the basal grain‐size and median thickness is different for turbidites with a basal A or B division and those with only C, D and E divisions. These two types of turbidite have been termed ‘thin bedded’ and ‘thick bedded’ by previous workers. A change in the power‐law exponent is proposed to be related to: (i) a transition from viscous to inertial settling of sediment grains; and (ii) hindered settling at high sediment concentrations. The bimodal thickness distribution of ‘thin‐bedded’ and ‘thick‐bedded’ turbidites noted by previous workers is explained as the result of a change in the power‐law exponent. This analysis supports the view that A and B divisions were deposited from high‐concentration flow components and that distinct grain‐size modes undergo different depositional processes. Summation of log‐normal frequency distributions for thin‐ and thick‐bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power‐law trend. Thus, the occurrence of both log‐normal and segmented power‐law frequency distributions can be explained in a holistic fashion. Power‐law frequency distributions of turbidite thickness have previously been linked to power‐law distributions of earthquake magnitude or volumes of submarine slope failure. The log‐normal distribution for a given grain‐size class observed in this study suggests an alternative view, that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters, in addition to the settling velocity of the grain‐sizes present.

Petrology and Dispersal Pattern in the Marnoso-Arenacea Formation (Miocene, Northern Apennines)

ABSTRACT Paleocurrent, gross composition, rock-fragment data, and heavy mineral data from the Miocene Marnoso-arenacea Formation of the Northern Apennines suggest six source areas. Five are primarily extrabasinal (Alpine I and II; Apenninic I, II, and III) and one is intrabasinal. The identification of characteristic multicycle materials reinforces the idea that migration of foredeeps within a plate-tectonic framework occurred in the Apennines. Integration of petrofacies, paleocurrent, and sedimentary-facies-analysis data in the study of the Marnosoarenacea depositional system has proved a very powerful method for understanding the complexity of a deep-sea fan system.