Gyrolithes：A Characteristic Trace Fossil in Marginal Marine Sediments

Key words: 
• Gyrdithes /
• brackish lagoon /
• firmgrotund-controlled trace fossil /
• envirorrmental significance /
• Tarim basin

Abstract: Gyrolithes, a kind of vertical dwelling burrow, may be formed by ilaterally symmetrical animals with paired appendages and often had a brackish water origin. Abundant trace fossils of Gyrolithes were occurred in the upper part of Tataaiertage Fm.Lower Silurian of many wells from Central Tarim basin, where the sediments were extensively bioturbated by Gyrolithes animals and contained very rare trace fossils like Beaconites. Based on the sedimentological, geochemical and paleoecological features, Gyrolithes from Lower Silurian of Central Tarim basin is formed in brackish lagoon environment. This kind of trace is generally very sharp walled and unlined, reflecting the stable, cohesive nature of the substrate at the time of colonisation and burrow excavation. It remained open after the tracemaker vacated the burrow, thus allowing material from the succeeding depositional event to passively fill the open structure. If the burrow had been excavated in mud, the domicile would have collapsed upon burrow vacation, unless lined. All these features indicate that Gyrolithes is a kind of firmground controlled trace fossil.