Chandipur mud balls:an enigma

Abstract

Mud balls, commonly referred to as discrete, near spherical masses of soft but cohesive sediment that have been eroded and transported intact from their original site of formation. They are reported from a wide range of depositional environments, both in modern settings as well as in ancient records. Although different models are proposed for generation as well as orientation of mud balls in different depositional settings, some common prerequisite conditions can be surmised, which are- (a) generation of mud balls must require erosion of formerly buried and slightly compacted thick mud layer by some high energy flows, strong enough to excavate the buried mud layer, and (b) subsequent transportation and depositional mechanisms control the distribution and orientation of the mud balls. The studied area in Chandipur coast is characterized by the presence of a vast, gently dipping (<20 dip) tidal flat (almost 3.5 km wide) and a narrow beach (slope 4-90) (100-120 m wide), with an estuary at the north-eastern side. Overall, it represents a very low energy environment with low tidal intensity, unable to cut any distinct tidal channel and disrupted occasionally by some storm channels only. Mud balls, both armoured and unarmoured, of different sizes and shapes (majority of them are ellipsoidal, ovoid, triangular, square, rectangular, trapezoidal and spherical) are encountered within the studied stretch, preferably along the junction between the beach and the tidal flat as well as on the seaward side of the larger barrier bars. River borne sediments, dispersed through formation of riverine spits, eventually get modified to form shore parallel barrier bars and merge with the coast line after migrating over the vast tidal flat, under tidal influence mainly. These local barrier bars isolate some portion of the tidal flat to produce a temporary low energy, calm and quite environment which is demarcated as ‘Swamp’ on their landward side. In response to the invading transgression, the barrier bars migrate over the thick swampy mud deposited on their landward side, which being thixotropic, remains almost static and gets compacted. Eventually the inactive, previously buried, swampy mud gets exposed on the seaward side of the migrated bar and becomes prone to desiccation and erosion. These desiccated mud acts as the source of the mud balls in the area. During sub-aerial exposure, as in case of low tide periods, this thick mud unit gets desiccated. The next high tide easily separates the desiccated mud chunks and carried them towards shoreline. Various shapes among mud balls may be attributed to the initial irregular shape of desiccated mud chunks. Hence, the formation of mud balls, in this setting does not require any high energy flows to erode out formerly buried mud as proposed in most of the earlier works as a necessary prerequisite. During transportation over the tidal flat, these mud balls get rounded as well as armoured with readily available organic shells. Under the low energy setting of this area, these heavy, elongated mud balls should be transported as bed load or rolling load and accordingly long axes of these mud balls should be oriented shore parallel after final deposition. Chandipur mud balls contradict this general consensus too, as a large number of mud balls used to show shore perpendicular orientations of long axes, as generally expected in case of sediment gravity flows due to transportation as suspension load. But, in a low energy coast like Chandipur, frequent presence of such high energy sediment gravity flows are hard to apprehend. Critical analysis has revealed a distinct relationship between the shape of the mud balls and its ultimate orientation, where purely ellipsoidal mud balls showed mostly shore parallel orientation of their long axes, a vast number of mud balls with tapered ellipsoidal (ovoid) shape were found lying with their long axes at high angle or perpendicular to shore line. This peculiar orientation of Chandipur mud balls can be attributed to the asymmetric mass distribution along the long axis in case of tapered mud balls and the couple of forces imposed on them by consecutive swash and backwash. Their behaviour, on the sloping shore is almost similar to that of an egg rolling along a lowly inclined (<10ᵒ dip) plane. Sometimes surface irregularity also influences the final orientation of mud balls. Preserved mud balls were found within some trenches in the foreshore of Chandipur beach, which showed both shore parallel as well as shore perpendicular orientations of the long axis. Therefore, it can be stated that the Chandipur mud balls, clearly stand apart from mud balls reported from other areas, both in genesis as well as final orientation of long axes. Critical investigation of the interrelationship among the slope, shape and orientation is required to avoid misleading interpretations for environments as well as paleo-geography.