Structures and sand injectites in Oligocene deep-lacustrine deposits, Bach Long Vi island, Gulf of Tonkin, Vietnam: Integrating UAV imaging with modern field work methods

Located at the junction between the Beibuwan Basin and the Song Hong Basin in the Gulf of Tonkin, the Bach Long Vi (BLV) island offers a rare window into the Cenozoic geology of Indochina and surrounding region. The island exposes 5 km-long continuous outcrop of weakly to moderately dipping Oligocene deep-lake deposits along the coastline. Using integrated UAV imaging and modern field methods, we characterized the deformation on the island at unprecedented level of details, and then related these features to regional tectonic events. BLV's mudstone-rich succession was intruded by pervasive sand injectites which consist of three main types: linear dikes, irregular dikes, and irregular sandstone bodies. Linear dikes and irregular dikes are found throughout most of the island. Linear dikes are steeply dipping, trend dominantly ENE, and tends to be thicker close to the top of the island stratigraphy. Irregular dikes have contorted shapes both in plan view and cross-section, and often connect adjacent linear dike segments. Irregular sandstone bodies are located toward the highest part of the island stratigraphy, and are characterized by depositional sands modified by sand injection processes. Most of the sand injectites on BLV constitute an interconnected complex that formed at the same time, probably in the late Oligocene during syn-rift. Sand injectites are later cross-cut by NW-SE left-lateral and its conjugate NE-SW right-lateral strike-slip faulting. The inferred ENE-trending maximum principal stress and NNW-trending minimum principal stress of this deformation is compatible with the regional end-Oligocene inversion as indicated by seismic data.