**7. Paleo-dimensions, flow dynamics and paleogeography**

The quantitative parameters of Early Permian Barakar River in Rajmahal Gondwana basin of eastern India suggest systematic changes in paleo-dimensions and flow dynamics. The Brahmini sub basin in the south was 10.91–14.96 m deep and 817– 2956 m wide, carried coarse to medium grained sand (>700 microns) and flowed over a steep northward paleoslope of 6.5 to 16.2 � <sup>10</sup>�<sup>4</sup> through the alluvial plain; the channel belt width ranging between 6310 and 6780 m and drained an area of <sup>136</sup> � 103 to 107 � <sup>10</sup><sup>3</sup> square km. Shields stress, inversion estimates consistently show higher difference in paleoslope (i.e. higher slopes up dip and lower slopes down dip) relative to paleoslope derived from ref. [30] is, perhaps, due to the Trampush's method using a continuous function to estimate slope. Reconstructed Rouse numbers, (Z), indicate that dominant transport modes of bed material varied in space and time (**Table 1**). The proximal field sites consistently exhibit high Z values (between 3.58– 3.46), indicating predominant bed load transport. Sediment load parameter value is characteristic of a bed load channel where the bed load forms about formed only about 17.74%, whose mean annual discharge was 22,070 m<sup>3</sup> /sec, and mean annual flood was about 170,600 m<sup>3</sup> /sec. These quantitative estimates along with sinuosity value of 1.361–1.383 characterize low sinuosity braided stream channels (**Figure 3**). The Flow characteristics remained in the upper part of the lower flow regime (Froude number = 0.208) and flows tend to be dominated by laminar flow because the estimated value of Reynolds number (Rep) is low (**Table 1**); the flow velocity between 1.57 and 1.89 m/sec caused coarse to medium grained sand to be in a dune configuration. The power equations suggest that the bedform length varies from 34.21–25.52 m and corresponding dune height is 1.363 m, comparable in magnitude with large dunes.

As the Barakar River entered in Pachwara basin in the north, the length is reduced to 631 km which drained an area of 81,000 to 86,536 square km; it had a mean annual discharge of 12,865 m3 /sec and a mean annual flood of about 112,200 m<sup>3</sup> /sec. The river was about 697 m wide and its depth in between 9.64–13.87 m. This multithread stream with sinuosity of 1.517 carried 15.62 percentage of bed load flowed on a moderate slope of about 0.000144 with flow velocity between 1.35 and 1.77 m/sec. It is evident that the upper part of the lower flow regime caused the medium-coarse grained sand to be in a dune configuration attains a height of 1.247 m and bedform length in between 23.27–31.40 m (large dunes). Calculated high values of Rouse number, Z, indicates that dominant transport modes of bed material and in Pachwara sub basin implying that bed load remains the most important transport mode. A greater width/depth ratio and moderate sinuosity streams are known to transport little silt and clay in their channel perimeter. The trunk Barakar River in this sub basin with modal coarsemedium sand transported its load as bed load at the threshold of motion. Critical stream power in this basin decreases to 0.152 Pa to carry coarser sediments (D50 = 0.57 mm) and produce lower amount of sediment flux a common phenomenon in many modern natural rivers. This sub basin shows low paleoslope between 0.000052–0.000152 indicating that river channels carry more wash load relative to total load which, in turn, would imply less boundary shear stress (8.66 Pa) in these river channels. Further north in Chuperbhita sub basin, its length is decreased again to about 525 km, flowed down the paleoslope from southwest to northeast having drainage area of about 60,512–37,000 km<sup>2</sup> . The mean annual discharge was about 5880 m3 / sec during normal period and up to 65,450 m<sup>3</sup> /sec during periodic flood. Consequently, the water velocity of 1.12 m/sec through most time but increased to 3.01 m/ sec during flood and clast flow stages, respectively. The quantitative parameters provide evidence for a relative decline in the channel depth (8.58–12.56 m), width (534 m) and channel belt width (3240–4305 m) and meander lengths of 7230 m, and increase in channel sinuosity up to 1.62–1.66, implying a decline in the slope of the Chuperbhita sub basin as compared with the southerly sub basins (Pachwara and Brahmini). The river channel in this sub basin flowed over a gently sloping surface (0.000139–0.000045) with relatively lower flow velocity of 1.12 m/sec. Indeed, there is an increase in the channel sinuosity owing to an increase in fine clastics and decrease in the stream power as indicated by boundary shear stress and critical shear stress. The sediments so transported are medium to fine sand with more of fine clastics. These changes in the channel parameters together with a decrease in the bed load percentage (10.14) and Rouse number (2.01) are indications of sinuous stream. The hydrodynamic characters (Froude number = 0.174, Reynolds particle number = 29.573) indicated that the flow in the stream channel was tranquil and river flow was in the lower flow regime, which, in turn, gave rise fine- to medium grained sand to be in large and medium scale bed configuration of relatively lower height of 1.073 m and bedform length in between 26.97–18.36 m as compared to southern sub basins.

In the extreme north in Hurra sub basin, its length is further reduced to 427 km and drained an area to the north and northeast of 42,257–28,000 km2 . The average water depth and channel width was 7.25 m, 408 m and increased up to 10.87 and 1380 m, respectively during annual periodic flood. Consequently, the mean annual discharge that was 3890 m3 /sec during normal period may jump up to 48,640 m3 /sec during periodic flood. The water velocity is 0.92 m/sec during normal period and up to 2.84 m/ sec during flood season. The stream power is reduced so as the sediment load parameter (M) 6.92 and Rouse Number (Z) further decreased to 1.71, suggestive of the mixed load remains the dominant transport mode of the streams. These narrow streams with meander wavelength of 6576 m swept over a depositional surface sloping at the rate of 116–176 cm/km in northeasterly direction. However, the flow in the stream channel was tranquil and in the upper part of lower flow regimen as indicated by the flow dynamics (Froude number = 0.149, Reynolds particle number = 16.75) which, in turn, facilities development of cross-bedded units in the fine- to medium grained sandstones. These dune bed configurations so developed attains relatively lower height of 0.928 m and bedform length in between 23.33–14.16 m as compared to southern sub basins. The stream channel sinuosity increased up to 1.632–1.728, owing to an increase in the fine clastics, implying a further decline in the paleoslope. However, low sinuous Barakar River, like those of the Brahmini, Pachwara and Chuperbhita became moderately sinuous (? Meandering) streams equivalent to 'mixed load channels', which might be due to intra-basin differential subsidence. To corroborate inferred sediment transport modes, the graphic plots of Reynolds particle number, Rep as a function of Shield stress, τ\* (**Figure 3**), are typical for bed load channel in the Brahmini and Pachwara in the south, whereas the mixed load realm for Chuperbhita and Hurra sub basins in the north. Down-dip, all data straddle the bounds between stability fields for ripples and dunes, which imply unidirectional flow and high to moderate sediment transport rates (both mixed transport and bed load transport).

Early studies have postulated northerly outlet of Permian rivers in this part of Peninsular India based on regional stratigraphic and sedimentary framework and

*Analyzing Sedimentary Rocks to Evaluate Paleo Dimensions and Flow Dynamics of Permian… DOI: http://dx.doi.org/10.5772/intechopen.106994*

#### **Figure 4.**

*Paleogeographic reconstruction during early Permian of eastern peninsular India. Solid arrow indicated the outlet of Barakar River.*

paleocurrent studies [55, 56]. In addition, newly identified isolated sub basins of Lower and Upper Gondwana litho-units in the sub-surface in Purnea of India and Bogra in western Bangladesh constitute north-south oriented larger Purnea-Rajmahal-Bogra Gondwana Graben [57, 58]. The southerly provenance of subsurface Gondwana rocks in the northern part of Bangladesh, further, justify the northerly sediment transport in this part of eastern India [59]. **Figure 4** illustrates paleogeography of eastern Peninsular India within the framework of eastern Gondwanaland. It is suggested that about 2260 km Long Early Permian River flowed northward from the Fox Ridge and had its outlet towards the cratonic shore zone close to the Himalayan foothills in the north [60].
