Recognising the Givetian Taghanic Event in the Devonian Baltic basin and its importance as a high-resolution international correlation datum
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Author||Marshall, John۱; Plink-Bjorklund, Piret۲; Mark-Kurik, Elga۳|
|Holding Date||04 October 2008|
The Orcadian Basin in Scotland is a terrestrial basin that contains an excellent record of Mid Devonian palaeoclimate and lies on the same palaeolatitude as the Devonian Baltic Basin. A high resolution correlation has already been made between the late Eifelian Achanarras fish bed (Orcadian Basin) and the Kernave Member (Baltic). A second late Givetian ’Taghanic’ correlation tie has now been achieved between the basins.
The Taghanic Event and Onlap in the Orcadian Basin was a distinctive group of climatic events within, and immediately overlying, the Eday Marl Formation in Orkney. The Eday Marl is an arid sabkha system that contains a high resolution archive of climatic change controlled by the relative strength of the seasonal insolation. This includes episodes of basin flooding as shown by the presence lacustrine laminites, bedded evaporites, marginal sheet flood sands and marine influenced bioturbated sheet sands. These flooding events are intercalated with intense and sustained episodes of aridity at times of relatively low seasonal insolation. Cycle analysis demonstrates their climatic origin and shows that the event had a duration of some four 400 ky long eccentricity cycles. Spores from the base of the Eday Marl are from the ex 3 spore zone.
In contrast, the spore assemblage from the upper part of the Eday Marl reveals a marked collapse to a vegetation dominated by archaeopteridalean progymnosperms (Contagisporites optivus and Geminospora lemurata, i.e. the IM spore sub-zone). The same spore assemblage was found in the Küllatova Quarry, Estonia, coeval with the Lode Quarry, Latvia, yielding rich fish assemblage and fossil flora; both localities are in the Lode Formation (Gauja Regional Stage). The Eday Marl is a time of sustained cool aridity and coincident with a series of marine regressions forced by ?thermally driven expansion and contraction of the water column. The pattern of events within the Eday Marl can be correlated with the Taghanic levels in New York State, USA. In NE Latvia and SE Estonia the top of the Sietini Fm represents maximum regression of the Eifelian to Givetian deltaic system, and is marked by a quartz-pebble conglomerate.
The overlying Lode Fm reflects a relative sea-level rise, and a turn-around into an aggradational phase of the delta succession. Hence the main Taghanic Event level can be recognised as this sub-Lode regression and hiatus with the Taghanic Onlap as the overlying Lode transgression and can be used to identify the base of the new latest Givetian sub-stage