Continental rift-setting and evolution of Neoproterozoic Sindreth Basin in NW-India

Abstract

The Neoproterozoic Sindreth Basin, NW India, and its surrounding area represent a half graben structure situated between the undeformed Malani Igneous Suite (MIS) in the west and a corridor of coeval Cryogenian ductile deformation, anatexis and granite intrusion in the east. The main lithologies observed in the basin are conglomerate, fanglomerate, debris flow and lake deposits derived from a nearby continental provenance, intercalated with concurrent mafic and felsic lava flows. Based on geological traverses across the strike of the basin, we propose a three-fold classification comprising Lower Clastic Unit and an Upper Clastic Unit and a Bimodal (basalt–rhyolite) Volcanic Unit separating the two. Tilting due to basin inversion and faulting has been observed; however, the rocks are unmetamorphosed and show undisturbed primary sedimentary features. The stratigraphic record of the basin is characteristic for deposition and magmatism in a fault-related continental setting. Implications of the findings have been discussed in the context of Neoproterozoic crustal dynamics in NW India. This study provides conclusive evidence for a continental setting for Sindreth Basin evolution and contests the recent models of active subduction setting (either back-arc basin or accretionary sediments over a subduction zone).

Appendix: Description of the Sindreth lithostratigraphy

1.1 A1. Lower Clastic Unit (LCU)

1.1.1 A1.1 Conglomerate facies

Generally, the conglomerate of the LCU is very poorly sorted with a wide spectrum of clasts varying from sand to boulders (up to 50 cm; figure A1-a). The components are angular to subrounded and comprise usually metasedimentary rocks of the Sirohi Group (carbon phyllite, mica-schist, quartzite) and milky quartz, most likely derived from quartz veins within these units (figure A1-b). Although the composition and clast size of the conglomerate is highly variable throughout the whole unit, some parts of the conglomerate are monomict as they are almost exclusively composed of large clasts of angular carbon phyllites (figure A1-c). The components are supported by a red-violet coloured matrix which consists of fine-to-medium grained sand and detritic ferruginous material. The conglomerate is amalgamated, as individual beds are usually not distinguishable. A rare phenomenon is the presence of lenticular sandstone bodies (figure A1-d). Those wedge out laterally and show a thickness up to 10 cm and an extent of up to 1 m.

Figure 10

Conglomerates of LCU. (a) Unsorted, angular clasts of different size. (b) Rock slab showing polymictic clasts, milky quartz is well rounded. (c) Monomictic parts showing large angular fragments of metasediments. (d) Intercalated sandstone beds.

1.1.2 A1.2 Pebbly coarse-sandstone facies

The pebbly coarse-sandstone facies represents the upper 25 m of the unit (figure A2-a). This facies is intermittently interbedded with the conglomerates and shows a gradual increase in proportion towards the top of the LCU. The deposit cannot be confined to one specific grain size, but rather shows a variation of dominance of coarse sandstone and domination of fine pebble (figure A2-b). The geometry of the individual layers is sheet like, with a thickness generally <50 cm. The components are angular to subrounded and include quartz, feldspar, platy muscovite (up to 5 mm) and rock fragments. These comprise metasedimentary rocks (carbon-phyllite, mica-schist and quartzite), milky quartz and granites. The fabric is supported by a pale siliciclastic matrix. Radiometric analysis by Somani et al. (2012) and Scharfenberg et al. (2015) in the coarse-sandstone facies revealed an augmented natural-gamma ray activity due to an enrichment of uranium.

Figure 11

The pebbly coarse-sandstone, forming the uppermost part of LCU.

1.2 A2. Bimodal Volcanic Unit (BVU)

1.2.1 A2.1 Mafic volcanics

In between the ridges of the LCU and the rhyolites, the mafic volcanics form a depression. Outcrops are usually related to quarries. The mafic volcanics comprise basaltic flows (figure A3-a) and mafic volcanoclastics (figure A3-b). While mafic volcanoclastics are frequent in the basal parts of the sequence, the share of volcanoclastics decreases and basaltic flows dominate following up the stratigraphy. The maximum inferred stratigraphic thickness of the mafic volcanics is about $\sim$500 m. The mafics are tilted in west direction (mean dip direction of 285${^{\circ }})$, but show no evidence of further deformation. Epidote and olivine can be abundant, but are predominantly present as xenolithic agglomerates. The flows width lies in the range of 20–50 cm. Vesicles can be common and are usually unfilled. Some flows show spheroidal weathering (figure A3-c) which at places give a pillow-like appearance. However, also in the field it is evident that the rounded structures are secondary weathering effects (figure A3-d). According to geochemical analyses (van Lente et al. 2009), the basalts show a tholeiitic signature.

Figure 12

Mafic volcanics. (a) Massive basaltic flows. (b) Volcanoclastics are frequent in the basal parts of the sequence. (c and d) Vesicles and spheroidal weathering can be observed in some basaltic flows.

1.2.2 A2.2 Rhyolites

The rhyolites are the most significant topological feature in the Sindreth area. The felsic volcanics form a 250-m high north–south trending ridge (figure A4-a). The rhyolites have their maximum extension close to Sindreth with their maximum stratigraphic thickness of about 900 m. They thin out to north and are tectonically reduced to the south. The rhyolitic sequence varies strongly in its occurrence. However, it is possible to differentiate two main types: one shows small phenocrysts with a pale pinkish matrix (figure A4-c), while the other type is characterized by an almost granitoid texture showing large phenocrysts (up to 1 cm) and a dark red matrix (figure A4-b). The rhyolitic ‘layers’ show with 10 cm up to a couple of meters, a huge variance in their thickness and dip with steep angles to west direction. The whole sequence shows a quite regular joint pattern with predominantly subparallel joints dipping in west direction with 40${^{\circ }}$–60${^{\circ }}$. There is strong indication from magnetic fabric analyses that at least part of the rhyolites represent dyke intrusions which will be investigated in more detail in further studies. In the field it is hardly possible to discriminate if the Pamta ridge is an amalgam of rhyolitic flows and later intruded felsic dikes.

Figure 13

(a) The rhyolites build up the significant ridge of the Sindreth Basin. Rhyolites with (b) larger and (c) smaller phenocrysts can be distinguished.

1.3 A3. Upper Clastic Unit (UCU)

1.3.1 A3.1 Tuff facies

The tuffs are bright greenish-gray in colour, very fine grained and massive. They are usually silicified and can form small ridges ($\sim$15 m) which are aligned parallel to the general striking (N–S). The pale volcanic ash is often interlayered by small bands of darker shale giving a laminated appearance. The filigree bedding shows synsedimentary brittle deformation (figure A5-a). The thickness of the tuff layers can reach up to a few meters. In between these laminated beds, massive strata also can occur. These have cherty appearance due to intense silification (figure A5-b). In the lowermost $\sim$750 m part of the Upper Clastic Unit, the tuff beds are rarely interrupted by sand layers. With the onset of coarse sedimentation, the ash layers become less dominant and most often form beds with thickness <20 cm, although they may reach a few meters in the later parts of the sections. The felsic volcanics alternate with greywackes, arkoses and conglomerates.

Figure 14

(a) Filigree laminated and (b) massive tuff layers are dominant in the lower UCU.

1.3.2 A3.2 Lithic greywacke facies

The lithic greywacke facies is characterized by yellow-brown normally graded sandstones with a coarser (pebble) grained, massive bottom layer, which gradually passes into fine-sandstone towards the top (figure A6-a, b). Individual sequences reach a thickness of 20–50 cm and show a gentle wavy erosional base contact. The grading of the greywackes becomes weaker in upper parts of the unit. The sequences show amalgamation, so that distinction of individual layers is not possible. Components are subangular to subrounded and comprise quartz, feldspar and rock fragments, which include metapelite, quartzite, silicified tuff, Erinpura- and Mirpur-type granite, basalt, rhyolite and rip-up clasts of rhyolitic tuff. Quartz grains show a higher grade of rounding than other components. The greywackes show a siliciclastic matrix. The sequences are intercalated by thin layers (generally <20 cm) of tuff or shale.

Figure 15

Graded sandstones of the lithic greywacke facies.

Figure 16

(a) Conglomerate facies and (b) arkose facies of UCU.

1.3.3 A3.3 Conglomerate facies

The conglomerate of the Upper Clastic Unit is brown-yellow in colour and poorly sorted with grain sizes ranging from pebbles to cobbles (figure A7-a). Most of the clasts larger than 2 cm are rounded to well-rounded and comprise mostly milky quartz and quartzite, but also granite, rhyolite, basalt and rip-up clasts of tuff. The deposits are massive and show normal gradation from coarse sand to pebbles/cobbles. Individual sequences commonly range between 0.5 and 1 m. In contrast to the greywackes, the matrix is not silicified and rather soft. Imbrication fabrics are rarely preserved and do not deliver trustful paleocurrent directions. Apart from that, the conglomerate is lacking sedimentary features.

1.3.4 A3.4 Arkose facies

The arkose facies comprises well sorted massive, brown-red sandstone (figure A7-b). The layers show a maximum thickness of 2 m and are intercalated by conglomerate, lithic greywacke and tuff. They consist of quartz, feldspar and micas, the latter being concentrated at surfaces parallel to bedding. The cementing material is silicic and components are subrounded to rounded. Sedimentary structures are absent.