Mud-Filled Ophiomorpha from Upper Cretaceous Continental Redbeds of Southern Mongolia: An Ichnologic Clue to the Origin of Detrital, Grain-Coating Clays

Abstract

At Flaming Cliffs-a famous vertebrate locality in southern Mongolia - sand grains in red sandstones of fluvial and eolian origin within the Upper Cretaceous Djadokhta Formation are coated with detrital clay. The nearest correlative marine deposits are thousands of kilometers distant. Structureless sandstones with stringers and lenses of reworked caliche pebbles lie laterally adjacent to sets of eolian cross-strata up to 20 m thick. These pebbly sandstones, interpreted as the deposits of intermittent streams that occupied interdune positions, contain abundant Ophiomorpha. Burrows are 1 to 3 cm in diameter, with a knobby exterior. The traces occur not only as simple tubes, but also as Teichichnus-like spreiten, produced by the systematic upward and lateral migration of a spiral burrow. Spreiten can reach vertical extents greater than 3 m and widths of 30 cm. Although they commonly appear in stratal sequences that contain no mudstone beds, many of the burrows are filled by mud. The best modern analog is provided by freshwater crustaceans that excavate deep burrows to the water table in order to escape desiccation during drought. Mud was emplaced during fluvial flood events when the open, near-vertical shafts acted as storm sewers through the vadose zone. Spreiten probably were generated by systematic offsets from the original burrow axis during repeated re-excavations. One mechanism for emplacing detrital clays in sandstones calls upon infiltration of muddy water through the vadose zone during flood events. The burrow fills within the fluvial facies of the Djadokhta Formation provide corroborative, independent evidence for the passage of muddy water through the strata. Like the Callianassa burrows that fill with coarse, shelly debris during the passage of Caribbean hurricanes (tubular tempestites), burrows in continental settings also can provide unique testimony to important depositional events and early diagenetic processes.