Life in an active volcanic arc: Petrology and sedimentology of dinosaur track beds in the Jindong Formation (Cretaceous), Gyeongsang Basin, South Korea

Abstract

The Jindong Formation was deposited as a thick succession of fine-grained volcaniclastic sediments in a rapidly subsiding intra-arc basin. These sediments contain abundant dinosaur and bird tracks that occur on many bedding planes throughout the approximately 110 m of stratigraphic section measured for this study. The main lithologies in the Jindong Formation are ash-rich mudstone, siltstone, and very fine sandstone. Minor lithologies are coarser sandstone, conglomerate, carbonates, and ash tuff. The sandstones are feldspathic and lithic arenites and wackes. They contain plutonic feldspars, volcanic rock fragments, volcanic ash, and a variety of quartz types. All lithologies show evidence of extensive post-depositional alteration, including devitrification and replacement of detrital grains by authigenic minerals. The alterations are typical of burial metamorphism with high heat flow. Sediments of the Jindong Formation can be grouped into three facies associations. Association 1 (proximal) comprises 16% of the measured sections. It consists of various types of bedded, cross-bedded, laminated, and ripple cross-laminated sandstone and minor conglomerate. These are interpreted as fluvial or distributary channel and proximal lake deposits. Association 2 (medial) comprises 53% of the measured sections. It consists mainly of heterolithic couplets of very fine sandstone and mudstone that are mm to dm thick. They contain graded bedding, laminations, current ripples, wave ripples, and mud cracks, and are interpreted as lacustrine hyperpycnal flow deposits. Association 3 (distal) comprises 31% of the measured sections. It consists mainly of bedded shale, mudstone, and siltstone. These are interpreted as distal lake deposits. Associations 2 and 3 contain minor carbonates, which are interpreted as lacustrine and/or diagenetic in origin, and minor tuffs, which are interpreted as tephra. Small wave ripples, dinosaur tracks, invertebrate burrows, and mud cracks are common throughout the measured sections; thus the Jindong lakes are inferred to have been shallow and ephemeral in nature. Wave ripple-crest orientations are consistently WNW-ESE, indicating a prevailing wind direction from the NNE or SSW. Because the sediments are mostly fine-grained and the ash beds thin, the study area is inferred to have been in a distal lowland setting, at least tens of km away from erupting volcanoes. Paleoclimate reconstructions suggest that the study area had a seasonal climate with wet summers and dry winters. Cooler, wetter conditions are thought to have prevailed to the north, and warmer, drier conditions to the south. Modern volcanic arc environments with seasonal and/or humid climates are characterized by rapid aggradation of ashy sediment that fills drainages and causes widespread flooding. This style of sedimentation accounts for the facies found in the Jindong Formation, as well as the many bedding-plane exposures of tracks. The unusual occurrence of both sauropod and ornithopod tracks in the Jindong sections may be the result of a climate boundary that existed in the area at the time. Facies analysis of track types indicates that various track makers had the same environmental preferences, and facies association 2 contains the greatest number of tracks of all types. It is inferred that the tendency for only one track type to occur on a given bedding plane is the result of different types of animals visiting the study area at different times, rather than differing environmental preferences. The Jindong Formation contains an abundance of subaqueously deposited facies, an abundance of tracks, and many track levels that were wet when animals were walking on them. These features are not consistent with previous interpretations of aridity during Jindong time.