ENVIRONMENTAL CONTROLS ON TAPHONOMY OF NEOPROTEROZOIC DOUSHANTUO PHOSPHATIZED ANIMAL EMBRYOS

Despite the significance and spectacular preservation of the animal embryos from the Neoproterozoic Doushantuo Formation of southwest China, a complete understanding of the paleoenvironmental conditions which led to the preservation of these magnificent fossils has yet to be reached. This research utilizes a combination of field and petrographic observations of the Weng’an Phosphate Member of the Doushantuo Formation in order to gain insight into the taphonomic processes involved in the preservation of these embryos. Results indicate that there are two distinct lithofacies within the Weng’an Phosphate Member: a lower black facies and an upper gray facies. Within each of these facies, phosphogenesis and phosphatization took place under different environmental conditions. Less reworking was involved in the deposition of the black facies, and dolomite is not present in the lower 2 m of this facies, which is rich in kerogen and pyrite. The gray facies is characterized by more abundant dolomite, primarily as a matrix for phosphoclasts, and greater levels of reworking. The transition from the black to gray facies involves increasing dolomite levels and decreasing kerogen levels. Based on these observations, as well as the position of the Weng’an Member directly above a subaerial exposure surface, the black facies is interpreted as an extremely stagnant shallow marine, perhaps lagoonal, deposit, that was deposited as sea level was just beginning to rise again, and the gray facies as a more open marine deposit. The environmental differences between these two members bear great taphonomic importance because of the resulting difference in reworking levels. The lower levels of reworking in the black facies, evidenced by poorly to very poorly sorted phosphoclasts up to pebble-size, would indicate the presence of a larger taphonomic window in this member as fewer phosphatized fossils would be destroyed during reworking. Fossil content of the two facies supports this assertion: while the gray facies is overwhelmingly dominated by large animal eggs and embryos, the black facies is characterized by a more diverse array of fossils including animal eggs and embryos, putative adult animals, acritarchs, and algae.