1,720,990 research outputs found
Dolomitization impact on fracture density in pelagic carbonates: contrasting case studies from the Gargano Promontory and the Southern Apennines (Italy).
No full textThe results of combined field and laboratory analyses carried out in well bedded, partly dolomitized successions of Mesozoic pelagic carbonates outcropping in the Gargano Promontory and Southern Apennines (Italy), revealed that dolomitization can exert opposite roles in modulating fracture density of carbonate successions.
The coarsely crystalline dolomites of the Gargano Promontory, which are characterized by planar-S to planar-E textures and some intercrystalline porosity (3.6–18.4%), are less densely affected by stratabound fractures (mostly joints) than the precursor micritic limestones. In contrast, the finer-crystalline, no porous dolomites outcropping in the Southern Apennines, which are dominated by non-planar-A textures, are more densely fractured than the precursor micritic limestones. Therefore, intrinsic textural parameters of dolomites, such as crystal size, texture and porosity played a prominent role in modulating stratabound fracture density. In addition, dolomites of the Gargano Promontory are thicker bedded than the precursor limestone beds due to their diagenetic homogenization, which is enhanced by absence of strongly stylolitized bed surfaces and marly interlayers. This contributed to reduce the overall density of the stratabound fractures formed later.
As density of stratabound fractures is considered a prominent factor affecting the large-scale porosity of carbonate rocks, the results of this study provide new insights on how dolomitization may either increase or reduce the quality of fractured reservoirs of geofluids through its multiple controls on density of such fractures
DEPOSITIONAL AND DIAGENETIC INFLUENCES ON THE SPACING OF SUB-PARALLEL, BED-PERPENDICULAR JOINTS IN BEACH CARBONATES (GRANADA BASIN, SPAIN)
No full textThe present contribution focuses on the control exerted by both depositional setting and diagenetic evolution on the spacing of sub-parallel, bed-perpendicular joints in shallow-water, layered carbonate rocks. In particular, the study is conducted on bryozoan-dominated, skeletal carbonate grainstones and rudstones cropping out in the Granada Basin, southern Spain. The study carbonates, Tortonian in age, were deposited within different sectors of a beach-like environment. The several carbonate lithofacies recognized, after field and optical microscopy analyses, are related to specific biological (e.g. flourishment of bryozoan colonies) and sedimentary processes (fair weather versus storm wave action). The results of quantitative analyses, conducted on representative samples, allowed us to determine the mean values of
grain size, grain sorting and grain shape. Taking into account the main diagenetic processes, cementation and dissolution, which affected the study carbonate rocks, we identify the principal controls on the resulting measured values of porosity and Uniaxial Compressive Strength. As a result, a key role was played by the mean grain size which, in turn, affected the diagenetic processes and the subsequent spacing of sub-parallel, bed-perpendicular joints. The depositional setting is identified therefore as the main control on grain size; in fact, the intrinsic environmental factors such as wave regime, water temperature and siliciclastic input influenced both the flourishment and growth form of the different biota and the sediment redistribution
The role of neptunian dikes on strain localization within layered Apulian carbonates, Italy
No full textOutcropping strike-slip faults in Cretaceous peritidal limestones (Altamura Fm.) display different type of calcareous breccia present either within isolated pockets or continuous volumes surrounding the main slip surfaces. Furthermore, original neptunian dikes and sheared neptunian dikes infilled with bauxite-rich matrix and limestone fragments are along walls and pavements of inactive quarries of the Altamura area (Murge area, Italy). In this work, we focus on the distribution, geometry, texture and composition of representative sheared neptunian dikes and tectonic limestone breccias to investigate the role exerted by Cretaceour neptunian dikes on nucleation and growth of strike-slip faults within tight layered limestones. In fact, during post-Cretaceous tectonics some of the 10’s of cm to m-thick neptunian dikes have been sheared, either right- or left-laterally, deforming its original fabric. At the same time, cataclastic deformation of intact limestones took place, at very shallow depths, within the evolving fault zones forming tectonic breccias.
The sheared neptunain dikes are sub-vertical open fissures infilled matrix-supported monomictic intraformational clasts embedded within a reddish hematite-bauxite clayish matrix mixed to calcite microsparry cement . Clasts are from cm- to dm-sized, sub-angular to angular and, overall, quite unsorted. In contrast, tectonic breccias consist of either isolated pockets (along m-offset faults) or continuous, at the mesoscale, volumes (10’s of m-offset faults) of cm- to 10’s of cm-sized, angular to sub-rounded clasts embedded within a calcite marix and often reddish bauxite matrix. Generally, the overall clast size decreases in the vicinity of the main slip surfaces. Meso- and microscopic evidence clearly indicate that clasts are due to comminution processes.
The results of mineralogic analysis conducted on selected samples of sheared neptunian dikes show that the red matrix and cements are mainly made up of quartz, calcite, kaolinite, hematite and goethite. Differently, the results of similar analyses conducted on samples of tectonic limestone breccias are consistent with calcite and iron oxides. The different mineralogic and textural composition of the two aforementioned elements well portray the variety of breccia distribution along the major, 10’s of m-offset fault zones present in the area. In fact the two different rocks show dissimilar grain matrix ratio and clast shape.
The conclusions of this study can be discussed in terms of the control exerted by pre-existing deformation on karst evolution. The study fault zone, actually, show evidences of multiple stages of karst development. Many features such as fracturing, brecciation and cementation suggest that subsurface fluid flow was strongly localized within these evolving fault zones. Faults with presence of remnants of former neptunian dikes can be inferred as a local barriers to fluid flow due to the presence of terrigenous materials chacterised by clay minerals. In contrast, faults in which the fault core is entirely made up of tectonic breccia the abundance of coarse and angular particles ensures efficient transmissibility
Faults and Fractures in Carbonates
Full text linkWorldwide, about 50 % of natural geofluids (i.e. mineral and
hydrothermal waters, geothermal fluids, oil and gas) are hosted
in carbonate reservoirs.1 Unlike other types of rocks, carbonates
consist of a great variety of lithotypes. Based upon the nature and
organisation/shape of the constituent elements (grains, pores, cement,
minerals, etc.), which are strongly related to the depositional setting
and in relation to their diagenetic evolution, carbonate rocks are
characterised by a wide range of porosity and permeability.2,3 Besides
the aforementioned primary controls on the petrophysical properties
of carbonate rocks, the containment and migration of geofluids in
carbonate rocks are strongly influenced by fault zones and fractures.
In the last few years, numerous researches were aimed at understanding
the faulting and fracturing processes in carbonate rocks, as well as the
quantification of their spatial and dimensional distributions in relation to
the physical-chemical properties of the various types of carbonates.
Following these lines of research, the Faults and Fractures in Carbonates
(F&FC) project of the University of Camerino, currently sponsored by a
consortium of oil companies, is aimed at significantly improving current
knowledge on the role of faults and fractures in the fluid flow properties
of carbonate rocks.
In several geological contexts, carbonate rocks are characterised by both
diffused and localised strain, which can be represented by background
deformation and fault zones, respectively. The combination of diffused
and localised deformation may form a discontinuity network that affects
the hydraulic rock properties. With regards to background deformation,
the primary control on fracture types, spacing and connectivity is
provided by the skeletal grain assemblage, heterogeneities such as
bedding or inclusions and lateral/vertical variations in cementation and
porosity. For what concerns the fault zones, in low-porosity carbonate
rocks they form preferential sites for fluid migration or accumulation,
16,17 whereas they represent barriers to fluid migration in high-porosity
carbonates.10,11,14 In the present contribution, we group the results
of our research activities into three main subjects: (i) background
deformation, in which we report an examples of studies aimed at
investigating the control exerted by host rock heterogeneity on
the development of background structures in layered carbonates;
(ii) faulting of high-porosity carbonates, characterised by a faulting
mechanism which determines both volumetric and shear strain
localisation into narrow tabular bands that constitute seals for fluid
flow;10,11 and (iii) faulting of low-porosity carbonates, in which we
report a synthesis of our studies focusing on the deformation
mechanisms, architecture, multiscale and petrophysical properties of
various types of fault zones
Tectonic vs. gravitational carbonate breccia within karstified strike-slip faults
No full textWe investigate the geometry, distribution, texture and composition of several samples collected along representative sheared breccia dikes and strike-slip faults crosscutting Cretaceous limestones. The study area is located in the Murge Plateau, Italy, where Santonian limestones pertaining to the Calcare di Altamura Formation is nicely exposed along the walls of inactive quarries. During post-Cretaceous tectonics, the 10’s of cm-to-m thick breccias dikes has been sheared, either right- or
left-laterally, deforming its original fabric. At the same time, cataclastic deformation took place, at very shallow depths, within the evolving fault zones brecciating the limestone rocks forming tectonic breccia. The former breccia deposits infill sub-vertical open fissures characterized, at the mesoscale, by a constant thickness. These deposits are made up of an uncemented, clast and matrix-supported breccia with reddish matrix, carbonate cements and iron-manganese oxides. Clasts are from cm- to msized, sub-angular to angular and not sorted at all. Tectonic
breccia, differently, consists of either isolated pockets (along moffset
faults) or continuous, at the mesoscale, volumes (10’s of m-offset faults) made up of cm- to 10’s of cm-sized, sub-angular to sub-rounded clasts embedded within a reddish microcrystalline matrix. Generally, the tectonic breccia is clast-supported. The overall clast size diminishes near the main slip zones. Meso- and microscopic evidences clearly indicates that clasts are due to comminution processes. Ongoing mineralogic analysis will shed lights on the nature of matrix and cements present within the investigated samples. The results of this study will be discussed in terms of the control exerted by pre-existing brecciated
limestone on karst evolution. In fact, the aforementioned breccias strongly affected the paleo-fluid pathways within the evolving strike-slip fault zones, as shown by the numerous field evidences documented in this work
Large-scale stratigraphic architecture and sequence analysis of an early Pleistocene submarine canyon fill, Monte Ascensione succession (Peri-Adriatic basin, eastern central Italy).
No full textThe Monte Ascensione succession (c. 2.65–2.1 Ma) is a well-exposed example of an exhumed submarine canyon fill embedded within slope hemipelagic mudstones. This gorge represented a long-lasting pathway for sediment transport and deposition and during the Gelasian delivered Apennine-derived clastic sediment to the adjacent Peri-Adriatic basin. A total of six principal lithofacies types, representing both canyon-confining hemipelagic deposits and canyon-filling turbidity current and masstransport deposits, can be delineated in the studied sedimentary succession. The canyon-fill deposits display a marked cyclic character and the component lithofacies succeed one another to form at least fifteen fining-upward stratal units, which are interpreted to represent high-frequency, unconformity- bounded depositional sequences. Variability in the vertical repetition of constituent lithofacies allows the identification of three basic styles of sequence architecture that can be interpreted in terms of differing positions along a conceptual down-canyon depositional profile. An integrated chronology, based on biostratigraphic data and on palaeomagnetic polarity measurements, strongly supports a one-to-one correlation between the sequence-bounding surfaces and oxygen isotope stages G2–78, suggesting that the most feasible sequence-engendering mechanism is that of orbitally dictated glacio-eustatic changes in sea level, which regulated timing of sediment storage on the shelf and its redistribution beyond the shelf edge. One of the most significant aspects of this study is the demonstration that processes occurring within upper slope canyons can be expected to be strongly influenced by variations in sea level; that is, the erosional and depositional features evident in these deposits can be strongly controlled by allocyclic processes rather than autocyclic or random processes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Evolution of the Gelasian (Pleistocene) slope turbidite systems of southern Marche (Peri-Adriatic basin, central Italy)
Full text linkExamples of slope channels and canyons being diverted by structurally related sea-floor topography are commonly found both on the modern seabed and in the subsurface, in deep-water fold and thrust belts but their depositional histories have rarely been documented using examples from outcrop. The synthesis of outcrop (original geological field mapping at 1:10,000 scale) and subsurface data sets from the early Pleistocene stratigraphic succession of the Peri-Adriatic basin provides a window into the overall evolutionary pattern of large intraslope turbidite systems that, during the Gelasian (2.58–1.80 Ma), shed Apennine-derived clastic sediments into the adjacent deep-water basin. Trends from mapping and paleocurrent indicators converge to indicate that the sea-floor bathymetric expression of a thrust-related anticline, the north-trending Jesi-Nereto-Zaccheo structure, likely influenced the downslope transport direction of gravity flows and sediment dispersal pattern. During early and middle Gelasian time, coarse-grained turbidite deposition occurred on the western flank of the intraslope anticline by westerly sourced, northward-flowing turbidity currents, indicating that the opposing sea-floor topography was sufficient to cause the diversion of turbidite systems, forcing them to travel near parallel to the east-facing regional paleoslope for significant distances. By very late Gelasian time, the intraslope accommodation space on the western flank of the anticline had filled and turbidites were dispersed through dip-oriented conduits incising across the crest of the underlying structure
Permeability structure of a >40 m-throw normal fault crosscutting a carbonate multilayer (SE Granada Basin, Betic Cordilleras, Spain)
No full textRecent road cuts expose the internal structure of normal faults crosscutting sandy/clay-rich conglomerates and
thin-beddedgrainstones. The detailed documentation of the deformation mechanisms associated to the processes
of normal faulting, carried out by mean of field structural analysis and laboratory investigations of representative
fault rock and host rock samples, is key to evaluate, at different scales, fault-related permeability structure of a
carbonate multilayer. In particular, we consider the role played by the different structural elements present within
the damage zone of a normal fault characterized by minor components of lateral slip and more than 40 meters of
throw.
The study area is located on the southeastern edge of the Granada Basin, named Tablate area, which consists of an
intramontane basin infilled by marine and continental deposits of Mio-Plio-Quaternary age. The whole basin infilling
is as thick as about 180 m and cover a Paleozoic-Triassic basement. Based upon their age, content and texture,
the siliciclastic/carbonate Miocene deposits are subdivided into four different members. The studied fault is made
up of an inner fault core flanked by faulted conglomerates (footwall) and thin-bedded conglomerates (hanging
wall), and crosscut basinal sediments gently dipping to the SW.
This study focuses on the fault-related structural elements present within the two lowest members, here named #1
and #2, respectively. Member #1 consists of brown-gray, clast-supported conglomerates arranged into 20 cm to 1
m-thick tabular beds with cm’s-sized schist and quartzite pebbles and a quite abundant sandy/clay gray-yellowish
matrix. The Uniaxial Compressive Strength (UCS), computed after Schmidt hammer testing, of the sandy portion
of the matrix is ∼21 N/mm2, whereas its 2D porosity ([U+F066]), calculated by image analysis of representative
thin-sections, is ∼10%. Member #2 consists of cemented grainstones(UCS= ∼70 N/mm2 ; [U+F066]=
∼5%) as thick as 40 meters. In the lowermost portion of this member, the grainstones are made up of rounded
bioclastic fragments and interbedded with cm-thick, greyish, clay-rich micro-conglomerates (UCS= ∼40 N/mm2
; [U+F066]= ∼7%).
Focusing on the main failure modes present within the damage zone of a normal fault active during the TortonianMessinian
age, which is characterized by more than 40m of throw and minor components of lateral slip, we document
contrasting deformation mechanisms. In the footwall damage zone that exposes the lowest member (#1), we
recognize pervasive shear banding and clay smearing along incipient and more evolved small faults with throws
up to 3m. On the contrary, in the hanging wall damage zone exposing member #2, joints and sheared joints are
present only in the carbonate beds, whereas stylolites and sheared stylolites localize in the clay-rich, thin microconglomeratic
interbeds. We infer that all these different structural elements formed, at the same time, during the
faulting of the carbonate multilayer. Specifically, we propose the following conceptual model for fault nucleation
and development:
- (member #1) in the conglomerate unit, normal faulting initiated by mean of two conjugate sets of shear bands
that formed thanks to grain rotation and translation. Ongoing deformation was accompanied by pronounced smearing
of the clay-rich portion of the matrix within the proto-faults, as well as by jointing of the individual pebbles.
Discontinuous slip surfaces formed at the hanging wall side of the tabular bands, which eventually linked together
forming through-going faults with m’s of throw and low-[U+F066] fault cores surrounded by almost intact conglomerates.
- (member #2) on the contrary, in the carbonate member incipient faulting occurred due to shearing across the preexisting,
overburden related, bed-perpendicular joint sets and formation of high-angle joints at their extensional
quadrants. In the micro-conglomeratic beds, when present, fault nucleation formed low-angle to bedding stylolites
at the contractional quadrants of the sheared bed-perpendicular joints. Continued slip caused the linkage among
structures present in neighboring beds, forming discontinuous slip surfaces surrounded by pods of fragmente
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