Abstract The Koyna borehole penetrated c. 1 km through the Deccan basalt units and into the cratonic basement beneath, thus providing a unique insight into the subsurface succession of the main Deccan province. Earlier studies focused on southwestern Deccan lava packages exposed in the Western Ghat escarpment, and resolved a well-constrained stratigraphy and key reference sections, but lacked supporting subsurface data. To construct the stratigraphy and correlate it with the main Deccan formations, we report flow-wise physical and chemical data of a c. 932 m-thick core. We document 37 lava-flow units and four lava-flow groups that have similar major-oxide contents. These groups fit into two of the recognized chemostratigraphic formations, and the transitional Poladpur–Ambenali lavas. In addition, data plots on Ba v. Sr; Ba v. Zr/Nb; Ba/Y v. Zr/Nb; and Ba, Sr, Ba/Y, Zr/Nb v. height bivariate diagrams confine them to the Poladpur and Ambenali formations. Lava flows match with the Khumbarli and Mahabaleshwar Ghat sections and Killari core. The granitoid basement–basalt and the Poladpur Formation v. Ambenali Formation contacts lie at −332.5 and c. 482 m above sea-level, respectively. Further, the new data endorse the southern overstepping of chemostratigraphic units and the asymmetry of the Deccan edifice due to the northward motion of the Indian Plate over the nascent Réunion plume ( c. 67–64 Ma). For comparison, the oldest 66.4 Ma lava flow predates the Cretaceous–Paleogene boundary (KPB) (66.052 Ma) by <0.35 Ma, with much of the Wai Subgroup erupted syn-KPB or >0.55 Ma post-KPB; however, the restricted lava thickness at the contact between the Poladpur and Ambenali formations provides a reference point in the Deccan stratigraphy.