4.1 Facies associations and depositional settings
The Barren Measures Formation in the Godavari River section is represented by two predominant facies associations, namely, a fluvial-dominated facies association (BM-F; Fig. 3) and a tidal-dominated facies association (BM-T; Fig. 4). Altogether eight facies types are identified within these two facies associations, appearing in cyclic repetition in the studied vertical succession (Fig. 2).
The fluvial-dominated facies association (BM-F) is characterized by pebbly, coarse-grained trough cross-stratified sandstone (BM-F1; Fig. 3a), planar tabular cross-stratified sandstone (BM-F2; Fig. 3b, c) associated with pebbly, coarse-grained plane-bedded sandstone (BM-F3; Fig. 3d) and red, ferruginous pebbly cross-stratified sandstone (BM-F4; Fig. 3e). The stacked, lenticular geometry of different cross-stratified sandstones with a concave-up base indicates the deposition from high-energy currents in channels. Concentrations of large pebble/cobble-sized clasts at the bottom of trough cross-stratified sandstone beds demarcate channel lag deposits (Fig. 3a). The predominance of coarse clasts with large trough cross-strata in the sandstones of the BM-F and relatively poor abundance of mudstones indicate channel-fill type deposition in a relatively high-energy river system. The poor sorting of the sediments with the mixing of coarse to fine, angular to rounded grains indicates short transportation from nearby sources.
The tidal-dominated facies association (BM-T) includes yellow sandstone facies (BM-T1; Fig. 4a, b), fine-grained cross-stratified heterolithic facies (BM-T2; Fig. 4c), red ferruginous siltstone/mudstone facies (BM-T3; Fig. 4d, e), and white calcareous mudstone facies (BM-T4; Fig. 4e). The facies types are characterized by a more sheet-like geometry, a relatively lower thickness and commonly represent the top of the fining-upward cycles. However, the sandstone-mudstone heterolithic facies and the mudstone facies become more pronounced and abundant towards the upper part of the overall succession (Fig. 2). Each facies type manifests definite signatures of tidal sedimentation in primary sedimentary structures. The yellow sandstone facies (BM-T1) and the fine-grained cross-stratified heterolithic facies (BM-T2) are characterized by laterally accreted foresets with mud drapes (tidal bundles; Fig. 4a, d), reactivation surfaces (Fig. 4a, b), lateral variations of the foreset thickness and frequency (Fig. 4a), cross-strata sets in opposite orientations (Fig. 4f), abundant flaser beddings (Fig. 4c), which unambiguously point towards the tidal control during deposition. Alternate traction and suspension deposition with thickness variation of the strata bundles manifest tidal fluctuations in different scales. All these facies types are characterized by intense cementation, both ferruginous and calcareous. Tidal bundles of different types are locally present within sandstone-dominated units in the facies BM-T3 (Fig. 4d). A massive to laminated white-grey calcareous mudstone facies (BM-T4) occurs at the top of this facies association and demarcates suspension fall out of finer clastics in a relatively restricted, calm and quiet environment, possibly in a stagnant water body. The overall nature of these facies types signifies a mixed tidal–fluvial depositional environment for this facies association (BM-T). The facies association is devoid of wave ripples, hummocks, etc. in the study area, which signifies the absence of waves during the sedimentation.
From the bottom to the top of the studied succession, more than twenty fining-up cycles with thicknesses between 5 m and 20 m were recorded (Fig. 2). Each individual upward-fining cycle is characterized by (i) coarse-grained pebbly sandstone of BM-F1 or BM-F3 grading upwards into relatively fine-grained sandstone/siltstone/mudstone of BM-T; (ii) other facies types, such as BM-F2, BM-F4, BM-T1, BM-T2, recurring in between the cycle; (iii) signatures of tidal influence increasing from the bottom to the top of each cycle. Such almost uniform characters of each fining-up cycle, with a distinct change of the depositional setting from fluvial-dominated to tidal-influenced, signify a retrograding characteristic within each cycle. Such a retrogradational stacking pattern points to repeated transgressive events in short intervals. The presence of the mudstone facies of BM-T4, though of varied thickness, marks the end of each fining-up cycle and possibly indicates the maximum flooding during each short-term transgressive event. The overall succession also depicts a fining-up trend, with the predominance of the coarser, channel-fill fluvial deposits (of BM-F) in the lower part, and finer, tidal-influenced marine deposits (of BM-T) towards the upper part. Relatively thicker fining-up cycles are observed in the lower part of the studied succession, whereas the upper part of the succession is characterized by frequent and thinner fining-up cycles (Figs. 2, 5a). Such a change from the fluvial-dominated to the tidal-dominated depositional setting via multiple transgressive events indicates a transgressive estuarine depositional setting for the studied succession. The absence of wave-generated features discriminates the possibility of wave reconstruction in such a mixed fluvio-marine environment, and points to a tidal-influenced, relatively protected, estuarine depositional setting.
4.2 Soft-sediment deformation structures
Different types of soft-sediment deformation structures (SSDS) are recorded characteristically from the basal coarse-grained sediment beds, mostly present near the lower part of many fining-up transgressive cycles. Meanwhile, SSDS with relatively smaller size and less abundance also occur in the finer-grained tidal deposits in some of the cycles. Such SSDS-bearing beds are commonly underlain and overlain by undeformed beds (Fig. 5b). These structures are unique because (1) most of them are much larger and of less comparable morphology which are relatively rarely described in the literature (see Alfaro et al. 1997, 2010); (2) these mostly occur within the pebbly, very coarse-grained sandstone; (3) large SSDS involve the deformation of several cross-stratified layers, where the normal grading from coarse pebbly sandstone to fine sandstone is present within individual foresets. However, large SSDS of varied origin are indicated by Alvarez et al. (1998), Debacker et al. (2001), Gibert et al. (2005), Alberti et al. (2017), and other researchers. Varied types of SSDS recorded in the study area are grouped and described under four distinct categories: (i) complexly deformed layers; (ii) load and flame structures; (iii) water-escape structures; and, (iv) syn-sedimentary faults. Brief descriptions of the salient morphological features of each category of SSDS are presented below.
4.2.1 Complexly deformed layers
Large complexly deformed layers are the most abundant type of SSDS in the study area (Figs. 6, 7, 8). They are characterized by broad troughs like vertical to inclined lobes with extremely folded layers (Figs. 6, 7, 8), separated by mushroom- or flame-shaped vertical-to-inclined narrow sediment bodies (Figs. 6a, b, d, 7a, c, d). The width and height of the larger lobes can range up to 2.5 m and 1.6 m (Fig. 7), respectively; and these lobes mostly develop within pebbly, coarse-grained cross-stratified sandstones involving thick, graded foresets in a large cross-strata set or multiple sets (Figs. 6d–f, 7a–d). Within the lobes, layers are highly deformed and are characterized by several smaller partitioned lobes and interlobes (Figs. 6a, c, d, 7a–c), symmetric to asymmetric pairs of lobes and isolated kidney-shaped bodies (‘pseudonodules’ of Owen 2003; Fig. 7d). The sediments in the interlobe flames show a complete mixing, as it ranges from pebbles to fine-grained sandstones and does not contain any preserved laminae. The folds on thick foresets are very smooth and show a thickness variation of the deformed layers from below the trough to the fluid-escape channel (Fig. 7a, d). Locally, large lobes of complexly deformed layers are preserved, which contain foresets deformed with simple, complex and overturned folds within the cross-strata sets (Figs. 6b, f, 8a). Relatively smaller lobes (with width of up to 0.8 m and height of 0.5 m) are observed in fine-grained heterolithic facies, where again the foresets in cross-strata sets are extremely deformed (Fig. 6c). Most of the deformed layers are truncated at the top, commonly by an erosional surface or by deposition of completely undeformed beds (Fig. 8). Though the coarse-grained sandstone beds bearing such large complex deformations are laterally persistent in the study area for more than 50 m, the intensity of deformations of the foresets varies laterally and may range from negligible to extreme deformation (Fig. 6f). Locally, foresets within thick coarse-grained cross-bedded sandstones are deformed to varying degrees producing overturned (Fig. 9a), partly contorted (Fig. 9b), ‘S’-shaped (Fig. 9c), or completely homogenized (Fig. 9d) cross beddings, bounded above and below by undeformed topset beds.
4.2.2 Load and flame structures
Simple load casts ranging in size from 15 cm to 35 cm are present in coarse-grained to relatively finer-grained sandstones (Fig. 10). These load structures appear as large lobes of coarse-grained, poorly-sorted sandstones loading onto the underlying medium- to fine-grained sandstones (Fig. 10a). Laminae in both beds are preserved and gently deformed, conforming to the load structures. Narrow flames in between the loads are of shorter length and height, consist of finer sand and are generally broad-crested (Fig. 10b). Sharp-crested flames are absent, possibly due to the absence of mud. Locally, the flame structures are preserved in foresets of large cross-strata, showing gradational transition from non-loaded to completely-loaded structures.
4.2.3 Water-escape structures including sedimentary dykes
Various types of water-escape structures are present within the thick cross-bedded coarse-grained sandstone facies (Fig. 11). Most commonly, these structures are represented by the vertical to inclined columns of coarse- to fine-grained sandstone and/or siltstone formed by the upward movement of sand/silt piercing through layers forming sedimentary dykes (Fig. 11 a-d). The upward drag of fluids moving through such columns has caused an upturning of beds/laminae adjacent to the columns (Fig. 11a). Locally a stacking of such upturned concave thin laminae has resulted in dish structures (Fig. 11b, d). Occasionally, sand-dominated vertical protrusions of irregular shapes sharply pierce through thin laminae in a discordant manner, producing multiple parallel sand flows (dykes) of relatively small dimensions with a width and height varying of 6–15 cm and 65–95 cm respectively (Fig. 11b, d). In some cross-stratified sandstone beds, a series of water-escape structures has caused contortions of the foresets. Locally, the water-escape columns are absent and instead a series of bulging successive sandstone layers caused by water escaping upwards, leading to a stacked inverted ‘V’-like deformation and producing the chevron bending of several layers together (Fig. 11c, e, f). The chevron structures persist laterally up to 20 cm and vertically up to 40 cm in a particular location. The position of the chevron structures locally shifts laterally and vertically within the thick foresets of cross-stratified sandstone (Fig. 11e). Such chevron structures are often truncated sharply by the overlying foreset laminae (Fig. 11f).
4.2.4 Syn-sedimentary faults
Meter-scale faults confined to specific layers are frequently present in the sandstone–mudstone heterolithic facies, causing truncations and displacements of the deformed laminae (Fig. 12a). The fault traces are 60–125 cm long in vertical sections showing steeply to gently inclined fault planes. Both normal and reverse faults are present. The faults die out in both upward and downward directions, show upward branching, and manifest greater throw near the base, which is changing to lesser throw upwards (Fig. 12 b-d). These properties of the faults characterize them as growth faults. Locally, the faults have been used as effective pathways of sediment–water mixtures for their upward moving, then producing complex water-escape structures along the fault planes (Fig. 12 b-d). In the study area, these smaller growth faults do not show any direct relation to the basinal faults. However, the formation of exposure scale small growth faults are very common in similar fault-controlled basins.