Located on the northern Vietnam onshore/offshore transition, the Hoanh Bo Trough is an excellently exposed terrestrial Palaeogene sedimentary sequence that may be treated as an analogue for regional interpretations of the sedimentary and structural evolution of the northern Song Hong Basin. The Hoanh Bo Trough lies to the north of the northern Song Hong Basin and to the west of the Beibuwan Basin, the origin and evolution of which are linked with Palaeogene South China Sea rifting. Field and archival well sedimentological observations were made throughout the Palaeogene succession of the Hoanh Bo Trough, and samples were collected for palynological, petrographical, and geochemical analysis. Based on the coexistence of particular lithofacies, proximal alluvial fan, distal alluvial fan, fluvial alluvial plain with channels, alluvial plain and/or lake margin, and lacustrine facies associations were distinguished. Palynological analyses suggest the sedimentary infill of the Hoanh Bo Trough is of the Late Eocene–Early Oligocene age and was deposited in a very warm tropical/subtropical climate. In turn, geochemical results demonstrate that the deposits have intermediate chemical maturity and were probably reworked from older sedimentary rock sources. Moreover, it is suggested to combine the Dong Ho and Tieu Giao formations and synonymize them as the Dong Ho Formation. The sedimentary pattern, age, climatic conditions, and structural evolution of the Hoanh Bo Trough align well with the rift initiation, rift development, and rift termination tectonic system tracts. Moreover, the Hoanh Bo Trough could be treated as a landward keyhole for the offshore basins: for instance, the Kien An Basin in the northern Song Hong Basin.

中文翻译：

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Hoanh Bo Trough - 北宋洪盆地（越南东北部陆上）同步裂谷晚始新世 - 早渐新世陆地层序的向陆钥匙孔

Hoanh Bo Trough 位于越南北部陆上/海上过渡地带，是一个出露极好的陆相古近纪沉积层序，可作为对宋洪盆地北部沉积和构造演化的区域解释的类似物。Hoanh Bo海槽位于宋洪盆地北部以北、北部湾盆地以西，其起源和演化与古近纪南海裂陷有关。在Hoanh Bo Trough的古近纪演替过程中进行了现场和档案井沉积学观测，并收集了样本用于孢粉学、岩石学和地球化学分析。基于特定岩相、近端冲积扇、远端冲积扇、带河道的河流冲积平原共存，冲积平原和/或湖缘，以及湖相组合被区分。孢粉学分析表明，Hoanh Bo Trough 的沉积填充物处于始新世晚期-渐新世早期，沉积在非常温暖的热带/亚热带气候中。反过来，地球化学结果表明，这些矿床具有中等化学成熟度，可能是从较旧的沉积岩源重新加工而成的。此外，建议将东河组和铁桥组合并，并将它们同义为东湖组。Hoanh Bo海槽的沉积模式、年龄、气候条件和构造演化与裂谷起始、裂谷发育和裂谷终止构造系统域吻合良好。此外，还可以将 Hoanh Bo Trough 视为近海盆地的向陆钥匙孔：例如，