Tidal Signature recorded by burrow fill: an example from the Miocene of Patagonia

WETZEL, A.; CARMONA, N.B.; PONCE, J.J.

Canakkale

Workshop; XII International Ichnofabric Workshop; 2013

The arrangement of sediment couplets preserved in Thalassinoides shafts suggests that tides regulated the passive filling of these trace fossils and thus, represent tubular tidalites. They occur in highly bioturbated sediments that accumulated around storm-wave base. The tubular tidalites preserve the so far only record of previous tidal characteristics within a Miocene embayment extending from the Atlantic to the West into northern Patagonia. The couplets consist of greenish terrigenous mud and whitish somewhat coarser sandy calcareous material. The greenish mud was deposited around high-tide slack water when preferably fine material settled to the seafloor. The whitish calcareous material is ascribed to be carried by currents, either flood and ebb current, while low-tide slack water was probably too short to result in the formation of a distinct layer. The tidal record within the burrows covers a few weeks and the corresponding spring-neap cycles. Two groups of couplets, thin-layered and thick-layered ones, are arranged in alternating packages suggesting a mixed semi-diurnal diurnal tidal setting as today. Around spring tide, thick-layered couplets dominate, suggesting a predominant diurnal tidal signature, whereas around neap-tide the semi-diurnal signature leads to the formation of two thin-layered couplets per day. The passive fill provides in maximum a record of 4 weeks documented within one example, whereas 3 other specimens contain intercalated unusual thick or even distorted layers and hence, restrict the undisturbed record to about 2 weeks. For the undisturbed intervals the sedimentation rate within the tubes ranges between 100 and 300 cm/yr. These values are in agreement with potential short-term sedimentation rates in modern tidal settings. The upper value refers to modern high-sediment yield depositional sites. The potential sedimentation rates shed some light onto the sediment dynamics in this part of the Miocene Patagonian shelf sea. The sediment fill of the Thalassinoides tubes always starts with thick-layered couplets and hence, suggest onset of fill around spring tide. This is suggestive that the behaviour of the Thalassinoides producers could have been affected by large water depth and low water temperature when abandoning their burrows. However, there are no supportive actualistic studies and hence, it remains a matter of speculation.