Liassic Limestones Research Articles

Superposed fracture networks

AbstractThe concept of superposed fracture networks consisting of different generations, and often types, of fractures that have developed sequentially is discussed. Superposed networks can consist of different types of extension or shear fractures, and each fracture may abut, cross or follow (reactivate) earlier fractures. An example of a superposed fracture network in Liassic limestones in Somerset, UK, is presented, which comprises two sets of veins and a later joint network. The veins develop as damage zones around faults, with veins of the later set crossing or trailing along the earlier set. The later joints either cross-cut the earlier veins or reactivate them, the latter being common for the thicker (more than about 5 mm) veins. The veins and joint networks have markedly different geometries and topologies. The veins are spatially clustered and are typically dominated by I-nodes, while the joints are more evenly distributed and tend to be dominated by Y-nodes. The combined network of veins and joints at Lilstock is dominated by X-nodes because so many joints cross-cut the earlier veins. Understanding the development of superposed fracture networks leads to better understanding of the kinematic, mechanical, tectonic and fluid flow history of rocks.

Complex monogenetic volcano in karst setting: Lechmine N'kettane volcano (Middle Atlas, Morocco)

The Lechmine N'kettane is a Quaternary volcano, located within the Middle Atlas Volcanic Field (MAVF) in central Morocco. It is built on the faulted contact between Liassic limestone and Plio-Quaternary fluvio-lacustrine deposits. In map-view it consists of an elliptical maar crater, surrounded by a tephra ring, within which a scoria cone is nested in its northern crater zone.The Lechmine N'Kettane volcano is monogenetic in the sense of its small eruptive product volume and lack of evidence of significant time breakthrough it grows. The volcano formed from an eruptive locus that migrated laterally and vertically within the short duration of the eruption in a zigzagging pattern, along a complex set of generally NE-SW and NW-SE-trending faults. It represents a perfect example of how a volcano form and evolve under the influence of a combination of specific factors such as the lithological characteristics of the substrate, its hydrogeological parameters, magma flux and the local structural framework of the country rocks.The petrographic, granulometric and morphological (including terrain modelling) analyses of the Lechmine N'kettane pyroclastic deposits show that it was constructed in four eruptive phases with variable eruptive styles. The first, relatively dry, phreatomagmatic phase, that took place on a NE-SW fault in the northeastern part of the crater, was generated by the interaction between the ascending basaltic magma with meteoric water in the karst aquifer hosted by the Liassic limestone. The second phase is represented by a magmatic scoria fallout deposit whose explosion locus moved westward, along the same NE-SW fault. During the third phase, the explosion center migrated southward, along a NNW-SSE fault, and produced the last phreatomagmatic event by interaction of magma and water-saturated Plio-Quaternary sediment. The fourth eruptive phase is a purely scoria event, corresponding to the construction of the nephelinitic scoria cone in the northwestern part of the tephra ring. Between eruptive products formed in respective eruptive phases no evidence was recognized to establish significant time gaps between their formation.

A Device to Register Temperature in Boreholes in Northwest Spain for Geothermal Research.

This paper presents a device used to measure and register temperature for long-term subsoil measurements in boreholes. The borehole of this study is located in Gijón (Asturias, Spain). The measurements were made through two fixed sets of sensors coupled to the geothermal pipe, constituting two independent installations: (a) a commercial device called “Hobo”, which uses TMCx-HD-specific sensors based on resistors with variable resistance; and (b) a device built by this research group, which uses DS12B20 Maxim sensors, a bus 1-wire, and a recording device based on a conventional Arduino board. Temperature was registered every 5 min across several years. These measurements were used to thermally characterize the subsoil, determining the apparent thermal diffusivity, and to study the thermo-hydrogeology of the Lower Jurassic Gijón’s formation made of Liassic limestones and dolomites. This work is part of the Q-Thermie group’s research called “Shallow Thermal Energy”.

Assessment of groundwater quality for drinking and irrigation in the Timahdite\u2013Almis Guigou area (Middle Atlas, Morocco)

The groundwater in the Timahdite–Almis Guigou area flows through fluvio-lacustrine and volcanic formations of Plio-Quaternary age and Liassic limestone. The groundwater resources in this area are used for drinking water and irrigation of agricultural plots. 18 groundwater samples were collected for this study. The physico-chemical and bacteriological parameters analysed, such as temperature, electrical conductivity, pH, dissolved oxygen, Na+, K+, Ca2+, Mg2+, Cl−, HCO3−, SO42−, NO2−, NO3−, faecal Coliforms, total Coliforms and faecal Streptococci, are used to characterise the quality of the groundwater and its suitability for drinking and for irrigation. The Piper and Gibbs diagrams and the saturation index were used to study the hydrogeochemical characteristics of groundwater. The quality of these was assessed on the basis of bacteriological quantification and water quality index (WQI) for drinking, and calculation of sodium percentage (Na%), sodium adsorption ratio (SAR) and permeability index (PI) for irrigation. The mineral saturation index in groundwater indicates that only carbonate minerals tend to precipitate, especially in the form of dolomite. On the other hand, the evaporative minerals are still undersaturated. The bacteriological quality and the water quality index (WQI) of this area are considered to be generally good to poor quality, with the exception of a few points, near the public dump of Almis Guigou, plateau of Lamrijate (Timahdite), Aït Hamza and Aït Ghanem, that show significant bacterial contamination and high concentrations of sodium, chlorides and nitrates. According to the calculation of the (Na%), the (SAR) and (PI), the groundwater samples are suitable for irrigation.

Formation of wide-blocky calcite veins by extreme growth competition

Liassic limestones on the coast of Somerset in the UK contain dense arrays of calcite microveins with a common, but poorly understood microstructure, characterized by laterally wide crystals that form bridges across the vein. We investigated the mechanisms of formation and evolution of these ‘wide-blocky’ vein microstructures using a combination of high-resolution analytical methods, including virtual petrography, optical cathodoluminescence and scanning electron microscopy techniques (e.g. energy-dispersive X-ray spectrometry, back-scattered electron imaging, cathodoluminescence and electron back-scattered diffraction), laboratory experiments and multiphase field modelling. Our results indicate that the studied veins formed in open, fluid-filled fractures, each in a single opening and sealing episode. As shown by the optical and electron back-scattered diffraction images, the vein crystals grew epitaxially on grains of the wall rock and we hypothesize that their growth rates differed depending on whether the crystals were on a wall rock grain substrate that fractured intergranularly (slow growth rates) or transgranularly (rapid growth rates). Our multiphase field models support this hypothesis, showing that wide, blocky crystals only form where there are significant differences in the growth rate and are dependent on the type of seed grain. These results provide strong evidence for extreme growth competition, a process that we propose controls vein-filling in many micritic carbonate reservoirs, as well as demonstrate that the characteristics of the fracture wall can affect the filling processes in syntaxial veins. Supplementary material: The description and images of the studied thin sections are available at https://doi.org/10.6084/m9.figshare.c.5172371 . High-resolution optical microscopy mosaics (under plane-polarized- and crossed polarized light) of the thin section collection in PetroScan file format are available on request from the authors.

Remote sensing, GIS, and Fieldwork Studies to assess and valorize groundwater resources in the springs area of oum-Er-rabiaa (moroccan Middle Atlas)

The basin of Oum-Er-Rabiaa is part of the Middle Atlas, it constitutes an important water resource on the scale of Morocco. The hydrogeological system of the region is characterized by many springs, some of which have a high salinity rate spilling into Oum-Er-Rabiaa river. The present study aims to investigate: Firstly, the distribution of lineaments along the major shear corridors crossing the area as well as their hydrogeological importance, and secondly, to verify the influence of geological characteristics on the physicochemical properties of the studied water bodies. This study is based on a combined analysis through: Remote Sensing, Satellite Imagery, Geographic Information Systems (GIS), Field Studies and Geochemical Analysis. The results demonstrated that the lithological and structural characteristics of the geological formations, play a predominant role in the creation of this hydrogeological system, as well as, in the diversity of the physicochemical proprieties of water resources in the area, these geological formations are mainly composed by: Liassic limestone characterized by a circulation of fresh water (fresh springs), this Liassic limestone is put in contact with the Triassic saline clays through geological faults, which is in the origin of the creation of saline springs contaminating the upstream of the Oum-Er-Rabiaa river. In the light of the decisive results obtained a set of solutions have been proposed such as the diversion of saline springs, the purification in situ, and the development of a fish farming project. These solutions aim to guarantee a better quality of water resources of the area, to preserve agriculture and to valorize the saline water bodies with a sustainable development perspective.

Phreatomagmatic plioquaternary volcanism in the Middle Atlas: Analysis of the eruptive sequence of the Lechmine n’Aït El Haj maar

Lechmine n’Ait El Haj (LNH) is a monogenetic plioquaternary maar, lying in the volcanic province of the MiddleAtlas. It is a 110-m-deep crater located in the Liassic limestones. The tephra deposits surrounding the crater are mainly made up of depositional units (surges and projectas) interpreted as deposits of phreatomagmatic origin. They are topped by a small unit of massive breccia tuff reflecting magmatic deposits. The maar is a result of the interaction between the ascending magma and karstic water, in an intraplate volcanism context. Water, causing this eruption, is drained by an open system of fractures in the limestone. The explosion started by phreatomagmatic dynamism, producing a big stack of pyroclastic deposits and pyroclastic falls. During the eruption, the crater grows progressively from the eruptive center to the Northwest. The upper part of the phreatomagmatic deposits is characterized by a typical mud crack structure. A transition to a strombolian dynamism occurred throughout the end of volcanic activity. Meanwhile, a lava flow, coming from the volcanic plateau, discharged in the crater’s center. With the eruption resumption, the lava is strongly fragmented; therefore, a small cone is created especially in the northern flank of the maar. Towards the end of the volcanic activity, a supply of karstic water causes another transition of the eruptive style from strombolian to phreatomagmatic dynamism. A significant karst collapse in the southern flank of the LNH maar has occurred, leading to its current morphology.

Strike‐slip reactivation of segmented normal faults: Implications for basin structure and fluid flow

AbstractReverse reactivation of normal faults, also termed “inversion”, has been extensively studied, whereas little is known about the strike‐slip reactivation of normal faults. At the same time, recognizing strike‐slip reactivation of normal faults in sedimentary basins is critical, as it may alter and impact basin physiography, accommodation and sediment supply and dispersal. Motivated by this, we present a study of a reactivated normal fault zone in the Liassic limestones and shales of Somerset, UK, to elucidate the effects of strike‐slip reactivation of normal faults, and the inherent deformation of relay zones that separate the original normal fault segments. The fault zone, initially extensional, exhibits a series of relay zones between right‐stepping segments, with the steps between the segments having subsequently become contractional due to sinistral strike‐slip movement. The relay zones have therefore been steepened and are cut by a series of connecting faults with reverse and strike‐slip components. The studied fault zone, and comparison with larger‐scale natural examples, leads us to conclude that the relays turned contractional steps are associated with (a) complex fault and fracture networks that accommodate shortening, (b) anomalously high numbers of fractures and faults, (c) layer‐parallel slip and (d) folding and uplift. Comparison with published statistics from global relay zones shows that whereas the reactivated relay zones feature aspect ratios similar to those of unreactivated relay zones, bed dips within reactivated relay zones are significantly steeper than unreactivated relay zones. Given the potential of reactivated relay zones to form areas of local uplift, they may affect basin structure and may also form potential traps for hydrocarbon or other fluids. The elevated faulting and fracturing, on the other hand, means reactivated relays are also likely loci for enhanced up‐fault flow.

New data on the Lower Mesozoic basal series of the Traras mounts (Tlemcen, northwestern Algeria)

The lower Mesozoic of the Traras mount starts with the so-called Red Series, which lays uncomformably on the folded and granitized Paleozoic basement. This series is overlain by Middle Liassic limestones. On the basis of its lower and upper limits and the lack of dating evidence, former authors attribute a Permian to lower Liassic (Sinemurian) age for this series. Palynological results allow to range it in the upper Triassic probably Norian-Rhetian. The Red Series is studied from five sections that depict important thickness and facies variations from southwest to the northeast, inferring the irregularities of the Hercynian erosional surface. In the Central Traras, this series shows over 130-m-thick micaceous sandy conglomeratic units, deposited within alluvial fans, where the bedrock source corresponds to granite of Nedroma or its equivalent. In the southern and eastern Traras, where the series’ thickness respectively does not exceed 30 m and averages 40–80 m, besides alluvial deposits issued from hercynian basement erosion, it exhibits volcanic evidences interbedded by lacustrine deposits. Vertical and spatial distribution depicts synsedimentary infilling of narrow elongated rift basins, typically half-grabens, bounded on one side by a normal fault or a series of normal faults trending NNE-SSW to NW-SE, inherited from the hercynian event. The study of tectonostratigraphic units permitted to put forward geodynamics of the infilling basins.

Role of Fracturing in the Organization of the Karst Features of Azrou Plateau (Middle Atlas, Morroco) Studied by Remote Sensing Imagery

Karst formation geometry can be controlled by fractures and faults, and by other lithologies. Here we show the organization of kastic collapse features related to structures and to extensive basaltic lava flows in the Middle Atlas of Morocco. A lineament map of major faults and fractures has been created for the Middle Atlas using Landsat 7 ETM+ satellite images. This shows a dominant NE–SW regional direction and less prominent NNW–SSE and ENE–WSE directions. All these directions coincide with the alignments of karstic depressions that have formed in the Liassic limestones. The basaltic flows covering these formations on the Middle Atlas limestone plateau, have allowed the generation of cryptokarst, geometrically organized a long these major lineament directions. Karst landforms probabaly existed before the eruption of the lavas, but there were partly invaded by intrusions and volcanism. The extensive basaltic flows allowed for increased infiltration, and subsurface water flow, increasing the rate of kast formation after eruptions. Some basins show evidence of increased subsidence after lava emplacement (Aguelmam Sidi Ali Lake) and some maar-like craters also have subsided after eruption, by karts formation. We lay out the structural and lithological controls on Karstic formation in an intraplate volcanic field based on limestones and evaporites.

Lithological properties and environmental importance of the Quaternary colluviums (Muğla, SW Turkey)

The colluviums are one of the common Quaternary deposits of the SW Turkey grabens. The protection of the colluviums is important for agriculture, ecology and safety of constructions. This study concentrates on the sedimentary properties of colluviums and environmental issues relating to them. The colluviums of the Mugla crop out in the northern-normal faulted margin of the Mugla Polje. These colluviums form wedge-shaped and narrow small strips (total 6 km in length, 250–300 m in width and 20 m thick), and contain four main facies delineated with respect to the field observations. These facies are A-Boulder facies, B-Cobble facies (gravel ≥94 % including cobble 10–46 %; mud + sand 17 %). Three representative sections were measured and 14 sieve analyses were made during this study. These colluviums include poorly to very poorly sorted angular limestone fragments bearing breccia. The colluviums are underlain by crystallized Liassic limestone and laterally passing into the alluvial sediments in a downdip direction. Tectonic activity, climate, gravitational forces and host rock characteristics are the primary controlling factors in the development of colluviums in Mugla. Agriculturally fertile soil derived from colluviums has decreased due to new residential areas and road construction. Moreover, excavations due to urbanization are causing many stability problems (landslides, debris flows, and rock fall–slides). The heavy rainfall and seismicity of the area increase the effects of these stability problems.

Characterization of proposed reservoir and seal rocks for CO2 geological storage in the Asturian Mesozoic sedimentary basin (NW Spain)

Abstract This study is focused on the investigation of four lithostratigraphic units (two potential reservoir rocks and two potential seal rocks) for geological storage of CO 2 in the Mesozoic Asturian basin (NW Spain). Two systems reservoir-seal are studied both in the Jurassic (Liassic limestones and dolostones of the Gijon formation as reservoir rock and Sinemurian-Bajocian marly limestones of Rodiles formation as seal rock) and Cretaceous (Aptian greywackes and sandstones of the Pola de Siero formation as reservoir rock and Albian grainstone-type limestones with argillaceous horizons as seal rock) sequences. Detailed information of the mineralogical distribution and texture, geochemical composition, porosity and pore size distribution is provided for each rock. Compared to the Cretaceous system, the Jurassic reservoir shows a better net thickness and pore size distribution and the Jurassic seal has an already demonstrated low permeability. The Cretaceous reservoir presents better porosity values (7.96% against a 0.5% of the Gijon formation), but the seal permeability is yet to be proven as low.

THE SCALING OF PULL‐APARTS AND IMPLICATIONS FOR FLUID FLOW IN AREAS WITH STRIKE‐SLIP FAULTS

Widths of pull‐aparts between strike‐slip fault segments appear to obey power‐law scaling relationships, at least up to the scale of hundreds of millimetres. Field data from the Liassic limestones of Somerset, UK, are presented and are discussed in relation to fluid flow through rock. The established scaling relationships are used to predict the contributions of different‐sized pull‐aparts to space created by strike‐slip faults. Fluid‐filled pull‐aparts are likely to be significant in hydrocarbon reservoirs affected by strike‐slip faulting because they are sub‐vertical tube‐like structures that may extend through reservoirs and top seals. There are, however, insufficient data to develop a model for permeability along pull‐aparts. Models of fluid flow in rock typically assume a limited range of fracture apertures, so models can be improved by incorporating information on the scaling relationships of the fractures and the contributions of different‐sized fractures to secondary porosity. Also, geomechanical models that assume planar strike‐slip faults are likely to underestimate migration because they do not consider the reduction of compressive stresses at pull‐aparts.

Sierra Elvira limestone: petrophysical characteristics of an Andalusian heritage stone

“Sierra Elvira stone” is one of the ornamental building stones most widely used in the historical monuments of eastern Andalusia. A Liassic age limestone, it appears in the central section of the Baetic Mountains and more specifically in the Middle Subbaetic domain. While the most common variety is a crinoid limestone, a micritic limestone of the same age has also been quarried, albeit in much smaller quantities. These stones form very thick beds, up to nearly 5 m deep, that run in consistently parallel lines and have a dip angle that facilitates quarrying. With petrographic, physical and mechanical properties that ensure stone strength and durability, it is a high quality building material suitable for both structural and ornamental purposes. These properties can be attributed to the minimal open porosity and concomitant excellent water resistance that characterize the stone, as well as to its high mechanical strength and low textural anisotropy, both elastic and mechanical. With such attributes, the stone can be successfully used for any number of purposes, including decorative stonework (portals, fountains, plinths), structural members (column shafts and bases) or urban curbing and paving. Intervention on Sierra de Elvira limestone structures should be limited to cleaning or repair, for consolidating or protective materials are scantly effective.

Fracture-pattern variations around a major fold and their implications regarding fracture prediction using limited data: an example from the Bristol Channel Basin

Abstract This chapter focuses on the evolution of fractures during the inversion of the Bristol Channel Basin, and examines the lateral and vertical consistency of the resulting fracture network within the alternating Liassic limestones and shales. The study has two principal aims. The first is to determine the reliability of fracture systems deduced using more limited data from less well-exposed regions or unexposed regions sampled only by drilling, and the second is to assess whether the fractures are linked to a regional stress field or are the result of a local stress field controlled by the geometry and mechanism of formation of a fold. The joint patterns were studied using a combination of scanline and window sampling, and the results indicate that there are considerable variations in the fracture systems between adjacent limestone beds and also lateral variation within the same bed. Although there is little doubt that the independent development of fracture patterns in adjacent limestone beds is facilitated by the intervening shale horizons, which allow them to become mechanically decoupled, the reasons for these variations are still unclear.

Differences between veins and joints using the example of the Jurassic limestones of Somerset

Abstract Although veins and joints are both extension fractures, they should be treated separately, especially in field analyses. For example, they commonly form at different times and under different conditions. Excellent exposures of the Liassic limestones at Kilve, Somerset, UK, are used as a case study. These rocks show calcite veins related to the Mesozoic extension and Alpine contraction of the Bristol Channel Basin, and joints that appear to have been caused by late- or post-Alpine stress release. The veins at Kilve show the following characteristics that distinguish them from joints. (1) They are mineralized. (2) Veins have measurable widths (up to tens of millimetres wide) and represent measurable strain. En echelon veins are common at Kilve, and indicate shear strains. (3) Vein widths may obey a power-law scaling relationship. (4) The veins were open for long enough and under the correct conditions for calcite to be deposited, indicating high fluid pressures. They are also typically en echelon. (5) Veins form sets with a narrow range of strikes, whereas the joints typically form complex networks. (6) The veins at Kilve are commonly clustered around faults, and are precursors to, or synchronous with, the faults.

Palaeostress orientation inferred from surface morphology of joints on the southern margin of the Bristol Channel Basin, UK

Abstract A field study of the orientation of surface features on joint planes has been carried out on the north Somerset coast along the southern margin of the Bristol Channel Basin (BCB), with the aim of inferring the palaeostress orientations. Surface features are very delicate structures on joint surfaces and when preserved give important clues regarding the direction of propagation of a joint and the interaction between the remote and local stresses. In this chapter surface features (i.e. plumose structures and twist-hackle fringes) observed on joints in Liassic limestone beds interbedded with shales from the BCB are described and their implications regarding the evolution of the stress field operating during their formation considered. The results are then compared with the generally accepted model for the region of an anticlockwise rotation of the stress field. The incompatibility between N-S-directed remote compression and continually changing local stress caused by the local structures (folds and faults), inhomogeneities and the anisotropy created by rapidly alternating limestone and shale beds resulted in the master joints breaking down into en echelon cracks (twist-hackle fringes). Joints in thicker limestone beds show well-developed plumose structures and twist-hackle fringes, whereas in thinner beds the fringe zones extend from the top to the bottom of the bed. It is found that regardless of the orientation of the parent joint in which the twist-hackle fringes are contained, they generally strike E-W and show both right- and left-stepping arrangements.

Present recharge of an aquifer in a semi-arid region: an example from the Turonian limestones of the Errachidia basin, Morocco

The Errachidia basin is composed of three superposed aquifers (Senonian, Turonian limestones and Infracenomanian). The Liassic limestone of the upper Atlas borders the northern part of the basin. The piezometric map of the Turonian aquifer displays a north-south flow, with an inflow area from the Atlas. This recharge hypothesis is demontrated by a discriminant analysis performed on chemical data: the majority of the spots are of sodium choride and hydrogenocarbonate types, while several boreholes are assigned to a calcium hydrogenocarbonate type Jurassic component. 18O measurements, using the Atlasic gradient δ18O=–4.18–0.0027 x elevation to estimate the recharge areas, confirm that the recharge area is the basin itself (<1100 m) on the Turonian outcrops, while in the confined part, the Turonian is recharged higher than 1400 m (corresponding to the Atlas). This contribution ranges from 56 to 85%, according to the situation versus the piezometric inflow area. The remainder represents infiltration and vertical leakage from the Senonian layers.

Hydrochemical Approach to the Alterations of the Recharge of a Karst Aquifer Consecutive to a Long Pumping Period: Example Taken from Pinchinade Graben (Mouans‐Sartoux, French Riviera)

AbstractThe carbonate aquifer of the Pinchinade graben, which has a well‐delimited geometry, involves water with well‐defined chemical types from calcium sulfate to magnesium bicarbonate. Each ground water type corresponds to a well‐delimited area of the aquifer: the magnesium bicarbonate for the Liassic and Rhetian limestone water (with 10 to 30 mg L‐1 of sulfate) and calcium sulfate for the water of the relatively impermeable layer of the underlying Keuper (with 300 to 1500 mg L−1 of sulfate). A four‐year pumping test with a bimonthly to daily monitoring of water chemistry has allowed evaluation of the renewal of the exploitable water reserve. During the four‐year period, the survey shows that the average discharge is balanced by natural recharge (2.8 to 105 m3 y−1). A change in the chemical character of the water was observed from Rhetian to Keuper type (from 35 to 167 mg L−1 of sulfate). Such a change indicates a progressive exhaustion of the Rhetian reserves, which are the greater part of the exploitable reserve in the area. The same phenomenon is observed daily depending on the pumped discharge and the season. For pumping rates below 26 m3 h−1, the borehole drains the Rehetian inflows to a degree depending on high or low water levels. For pumping rates above 26 m3 h−1, whatever the period, the permeable Keuper layers are pumped and sulfate peaks ensue.

Fracture surface markings in Liassic limestone at Lavernock Point, South Wales

Abstract A study of fracture surface markings (FSM) on three joint sets, striking at 190, 335 and 280°, cutting a 2.5 m thick slice of interbedded limestones and mudstones of lower Liassic age, yields evidence which enables determination of the orientation of the palaeostress field at the time of joint formation. A specific element of the FSM, namely the border planes, or the twist hackle, has been used to determine the orientation of σ 1 , in that these twist-hackle structures are considered to be developed normal to the remote tensile stress σ 3 . In the case of the joint sets striking at 335 and 190°, the sense of rotation of the twist plane would suggest that for both sets σ 1 was aligned N-S and almost parallel to bedding. Other FSM features suggest that the 190° set propagated rapidly with S-type plumose markings being the most common. The small angle of border plane rotation (10–15°) suggests that all of the joint sets propagated under relatively low confining pressures. The sense of border plane rotation on the 335 and 190° sets suggests that there is a possible conjugate relationship between them. However, the other main joint set which strikes at 280° cannot be accounted for by simultaneous development in a N-S aligned σ 1 which might have caused the development of the 335–190° joint sets.