143 / 2022-02-26 19:27:03

Ichnological, palaeoenvironmental, and high-resolution sequence-stratigraphic analysis of the Palaeosol Member from the Ypresian (early Eocene) Naredi Formation, western Kutch basin, Gujarat, India

process ichnology,root traces,Camborygma,lungfish aestivation burrow,Skolithos,Macanopsis,laterite horizon,palaeosol,high-resolution sequence-stratigraphy,Eocene Kutch basin

The Kutch Basin is a pericratonic rift basin, developed in a partially restricted-marginal to shallow-marine and extending upto a supratidal basinal setting at places. This basin accommodates the Mesozoic siliciclastic and the Cenozoic mixed siliciclastic-carbonate megasequences, separated by the Deccan volcanic and volcaniclastic deposits. The present study involves a detailed process ichnological investigation of the topmost palaeosol interval (the Palaeosol Member) of the early Eocene (Ypresian) Naredi Formation outcrops found in and around the western Kutch district of Gujarat state. The lithology of the studied interval is defined by the extensively exposed intercalation of the pedogenized beds of ferruginous laterite and silty shale (about 4.5 m thick). The laterite and silty shale beds thicken upward. This specific member is characterized by cyclic episodes of erosion, subaerial exposure, and deposition of high-resolution or low-order (i.e., more than or equal to 5^th order) sequence-stratigraphic hierarchy.

The punctuated lateritization events led to the development of several major ferritic laterite horizons, which include hard laterite, soft laterite or mottled clay zones, and saprolite layers as identified from the micromorphological features (e.g., coarse and fine fractions, undifferentiated micromass, nodules, and voids). The topmost and the thickest laterite bed represents intense to complete bioturbation (Bioturbation Index, BI = 5-6) of, in decreasing abundances, root traces, Camborygma isp., vertebrate (possibly lungfish) aestivation burrows, and Skolithos isp. as well as Macanopsis isp. (possibly arachnid burrows). The root traces show its characteristic shape with the downward diverging branches with ferruginous coating and filling. The Camborygma burrows, presumably produced by the terrestrial decapod crustaceans (crayfishes), can be defined by a simple to complex burrow system with well-defined sub-vertical shafts, sub-horizontal tunnels, chambers, and bioglyphs (scratch marks and striae). The aestivation burrows exhibit simple, smooth-walled quasi-vertical morphology with asymmetrical bulbuous chambers containing the characteristic quasi-horizontal striae on the wall and quasi-vertical striae concentrated near the terminus. Two types of arachnid burrows are observed with vertical to sub-vertical cylindrical shafts (Skolithos isp.) colonizing inside the lumen of the vertebrate aestivation burrows, thereby forming a composite ichnofossil. A few of these shafts contain a basal chamber at the bottom of each individual (defining Macanopsis isp.) and also demonstrate characteristic lining and striae on the burrow wall.

The less-pedogenized silty shale beds have been characterized by moderate to high bioturbation (BI = 3-4) with the paucispecific assemblage of Camborygma isp. and root traces. The lithofacies contains evidences of subangular-blocky ped development and cutans along with micromorphological features such as peds, nodules, voids, micromass of undifferentiated to monosteriated B-fabric, coatings, and hypocoatings, therefore, bearing convincing evidence of subaerial exposure and palaeo-pedogenesis. Ichnotaxonomic analysis of Camborygma burrows leads to the identification of Camborygma symplokonomos and Camborygma araioklados, which further assisted in understanding the alluvial palaeogeographic context. The variation of ichno-abundance and morphological complexity of Camborygma isp. serve as a significant tool for the palaeoclimatic and palaeoenvironmental reconstruction, demarcation of the sequence-stratigraphic horizons, and fluctuation of the palaeo-watertable in response to the low-order allocyclicity.

The laterally variable abundance of bioturbation, low ichnodiversity, dominance of vertical domiciles, blend of vertebrates, invertebrates, and plant traces in a gleyey palaeosol, hence, can be defined as a pedogenized variety of Scoyenia ichnofacies – a characteristic firmground ichnofacies developed under alternating subaqueous and subaerial condition – although without any archetypal palaeosol ichnotaxon. Based on the evidences provided by trace fossils and palaeosol characteristics, the palaeoclimatic and palaeoenvironmental setting has been interpreted to be a hot and humid alluvial one, probably on an interfluve, with intermittent episodes of seasonal rainfall, inundation, and sedimentation of fine-grained clastics in a very shallow (close to the strandline) lacustrine or lagoonal depositional environment.

The top of the thickest lateritized horizon in the studied area demarcates the onset of Lutetian sequence boundary, which is a subaerial unconformity separating the progressively shallowing-up and aridifying-up Palaeosol Member of the Ypresian Naredi Formation below and the fully marine Bartonian Harudi Formation above. The Harudi Formation marks the initiation of the Bartonian transgression that has been recorded globally.