Dissolution seams: some observations from the Proterozoic Chanda Limestone, Adilabad, India

Abstract

The Proterozoic muddy carbonate turbidite beds in the Chanda Limestone, India are invariably differentiated gradationally into a lower seamed and an upper nonseamed layer. Inherited seamed and nonseamed layers within the autoclasts constituting the basal pebbly and bouldery mass-flow deposits indicate preburial origin of the seams. Upward increase in the thickness ratio between the intrabed seamed and nonseamed layers and general enlargement of the seams in the transgressive sequence preclude any correlation between seam generation and building of the sediment overburden. Close δ¹³C and δ¹⁸O values obtained from the seamed- and nonseamed-layer couplets also exclude difference in diagenetic fluid composition as the cause for decoupling of every turbidite bed into layers of contrasting fabric. The paper lends support and suggests tangible modification to Eder's (1982) model for depth-related differential dissolution of seafloor carbonates, and extends that to explain the origin of the bed-parallel pre-burial seams. Lack of bioturbation and likely starvation of Precambrian deep carbonate basins should have favoured generation and preservation of pre-burial dissolution seams.