The fine-grained cyclic deposits of Olaipadi member of the Dalmiapuram Formation, Cauvery basin, contain large boulder sized lithoclasts of gneissic basement rocks and older sedimentary rocks. Earlier these were interpreted as glacial drop deposits and siliciclastic coral reef complex, etc. Many sedimentary characteristics, typical of these deposits are neither been documented nor explained by previous studies. They are: a. occurrence of large, genetically unrelated lithoclasts in typical basinal sediments that show a general fining upward nature; b. occurrence of conformable bedding planes along the margins of these large extraformational and extrabasinal boulders; c. repetitive occurrences of classic Bouma sequences; and d. sedimentary structures in the bases of sandy and silty layers indicative of fluidized flow and post-depositional dewatering, general reduction of thicknesses of sandy layers from basal beds to top and coeval increase in thickness of claystone layers. Examination of new exposures helped documentation of sedimentary textural, structural and lithofacies characteristics, contact relationships, facies association and tectonic structures. It is inferred that sedimentation of Olaipadi deposits was initiated by hyperpycnal flow following faulting and upliftment of former shelf and coastal regions that led to the gravity flow of fractured and unstable fault scarp blocks resulting in the turbidite deposition on the adjacent deeper regions of sea. Maturation of depositional topography from rugged fault controlled to gentle slope took place concomitant with progradation of fan into deeper basinal regions along with sea level oscillations.

The sedimentary terrain of erstwhile Tiruchirappalli district, South India, located in the Ariyalur-Pondicherry depression of the Cauvery basin (Figure 1) exposes a more or less complete Barremian-Danian succession (Sastry and Rao, 1964). Followed by the pioneering studies of Kay (1840) and Blanford (1862), many hundreds of papers were published on the lithological and faunal diversities of the succession and the possibility of locating huge hydrocarbon reserves. Among these publications, the works on foraminifera (Govindan et al., 1996), ostracoda (Bhatia, 1984), ammonite (Ayyasamy, 1990), nannofossil (Jafer and Ray, 1989; Kale and Phansalkhar, 1992; and Kale et al., 2000), bryozoa (Guha, 1987; and Guha and Senthilnathan, 1990 and 1996), lithostratigraphy (Ramanathan, 1968; Banerji, 1972; Sundaram and Rao, 1986; Ramasamy and Banerji, 1991; and Tewari et al., 1996) and tectonics (Kumar, 1983; and Prabhakar and Zutshi, 1993) present comprehensive accounts. However, owing to the poor and scarce exposures and comparatively less fossiliferous nature, the cyclic, regular bedded, fining upward sandstone-shale deposits of the Dalmiapuram formation (Albian-Cenomanian) have not been thoroughly studied. The presence of huge limestone blocks (~10 m dia) of shallow marine origin with basement rocks and lithoclastic boulders of similar size embedded in basinal sediments added many genetic controversies like glacial dropstones (Sundaram and Rao, 1986) and mixed coastal siliciclastic-carbonate reef systems. Furthermore, these models have not explained many characteristics of these deposits namely: absence of major glacial activity during deposition and occurrence of lithoclasts of conglomerates that were not recorded in any of the older deposits and large basement rock blocks. Thus the sequence remained less understood. The access provided by exposures from new mine sections, expansion of old mine sections, trenches excavated for construction of new bridges and road sections and traverses at closer intervals helped document a detailed micro-scale lithological, sedimentary structural and facies interpretations, thereby improving our understanding on the cyclic deposits.