Kačák Event

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The Kačák Event (Czech pronunciation: [kat͡ʃaːk]) or Kačák-otomari Event is a widely recognised bioevent or series of events that occurred close to the end of the Eifelian Age of the Middle Devonian Epoch. It involved a global eustatic rise in sea-level. It was named for the Kačák Member of the Srbsko Formation in Bohemia, where it is represented by a black shale interval within a sequence of limestone. In marine environments, this appears as an anoxic event, often forming potential hydrocarbon source rocks such as the Marcellus Shale. Within the Old Red Sandstone continent, it is represented by the Achanarras lake, the deepest and most widespread lake that developed within the Orcadian Basin. The event is associated with significant extinctions, particularly amongst the Ammonoidea.

Age and duration[edit]

The event occurred towards the end of the Eifelian, extending into the earliest part of the Givetian, in the mid-Devonian period. The duration of the event has been estimated as 700,000 years from the seven cycles involved in the Achanarras interval (each cycle interpreted to represent 100,000 years),[1] but only 200,000 years using geochemical and magnetic resonance data from the Eifelian-Givetian boundary in Morocco.[2]

Occurrence[edit]

The Kačák interval was first named from its occurrence in Bohemia as a black shale unit, known as the Kačák Member. This organic rich shale is found within a sequence of shallow water limestones forming the Srbsko Formation of the Prague Basin.[3] The event has also been recognised at Eifel in Germany, Gorodenka (near Omsk, Russia) and in Ontario and New York State in eastern North America, all localities lying on the then continental shelf around the Old Red continent. Further afield at the time of the event, on the continental shelf on the other side of the Rheic Ocean, it has been recognised in Morocco, Cantabria in northern Spain, the Carnic Alps and Graz Paleozoic in Italy and Austria, the Montagne Noire in France and in the Barrandian area of the Czech Republic. The Orcadian Basin of Scotland is the only locality within the continent itself that the event is recognised.[1][4] Similar events have also been correlated with the Kačák Event in China and Australia.[3]

Geochemistry[edit]

At the level of the Kačák Event there is a marked negative excursion in the δ13C level, interpreted to be a result of the anoxic event.[4] This reduction matches closely to an increase in both total organic carbon and a change in the fractionation of carbon between carbonates and organic 'reservoirs'.[5]

Cause[edit]

The cause of this event is interpreted to be a period of high temperatures resulting from high insolation levels. This explains both the transgressive event recognised at the margins of the Old Red continent, caused by thermal expansion of the oceans, and the formation of the Achanarras lake within the continent due to the increased intensity of monsoon conditions.[1]

Extinctions[edit]

The Kačák Event was a period of significant extinctions, although not as marked as those of the subsequent Late Devonian extinctions. The group most affected was the Ammonoidea,[3] although there were also faunal turnovers amongst conodonts and dacryoconarids.[6]

Economic importance[edit]

Organic-rich black shales that formed during this anoxic event occur in several countries. In the eastern United States the Marcellus shale is in the early stages of being exploited for shale gas, with large recoverable reserves predicted. An initial USGS assessment from 2002 suggested about 2 TCF of recoverable gas,[7] while in 2009, a study by the United States Department of Energy gave an estimate of 262 TCF.[8]

References[edit]

  1. ^ a b c Marshall, J.A.E.; Astin, T.R.; Brown, J.F.; Mark-Kurik E. & Lazauskiene J. (2007). "Recognizing the Kačák Event in the Devonian terrestrial environment and its implications for understanding land–sea interactions". In Becker R.T. & Kirchgasser W.T. Devonian events and correlations. Special Publications 278. London: Geological Society. pp. 133–155. Retrieved 3 March 2012. 
  2. ^ Ellwood, B.B.; Algeo T.J., El Hassani A., Tomkin J.H. & Rowe H.D. (2011). "Defining the timing and duration of the Kačák Interval within the Eifelian/Givetian boundary GSSP, Mech Irdane, Morocco, using geochemical and magnetic susceptibility patterns". Palaeogeography, Palaeoclimatology, Palaeoecology (Elsevier) 304 (1–2): 74–84. doi:10.1016/j.palaeo.2010.10.012. Retrieved 4 March 2012. 
  3. ^ a b c House, M.R. (1996). "The Middle Devonian Kačák Event". Proceedings of the Ussher Society 9: 79–84. Retrieved 3 March 2012. 
  4. ^ a b Kido, E.; Suttner T.J. (2011). "A new project has been launched: FWF P23775-B17 "Late Eifelian climate perturbations: effects on tropical coral communities"". Jahrbuch der Geologischen Bundesanstalt 151 (3–4): 407–416. ISSN 0016-7800. Retrieved 3 March 2012. 
  5. ^ van Hengstum, P.J.; Gröcke D.R. (2008). "Stable isotope record of the Eifelian-Givetian boundary Kačák-otomari Event (Middle Devonian) from Hungry Hollow, Ontario, Canada". Canadian Journal of Earth Sciences 45: 353–366. Bibcode:2008CaJES..45..353V. doi:10.1139/E08-005. Retrieved 4 March 2012. 
  6. ^ DeSantis, M.K.; Brett C.E. (2011). "Late Eifelian (Middle Devonian) biocrises: Timing and signature of the pre-Kačák Bakoven and Stony Hollow Events in eastern North America". Palaeogeography, Palaeoclimatology, Palaeoecology (Elsevier) 304 (1–2): 113–135. doi:10.1016/j.palaeo.2010.10.013. Retrieved 4 March 2012. 
  7. ^ Milici, R.; Swezey C.S. (2006). "Assessment of Appalachian Basin Oil and Gas Resources: Devonian Shale–Middle and Upper Paleozoic Total Petroleum System". Open-File Report Series 2006-1237. United States Geological Survey. p. 39. Retrieved 7 March 2012. 
  8. ^ US Department of Energy (April 2009): Modern shale gas development in the United States: a primer, p.17, PDF file, downloaded 7 March 2012.