November 2015 LIP of the Month

Late Ordovician basalts of Sierra del Tigre, Argentine Precordillera, and the Hirnantian mass extinction

G. J. Retallack, Department of Geological Sciences, University of Oregon, Eugene. Email:

Recent geochemical study [1] of Late Ordovician basalts in the Argentine Precordillera of San Juan Province (Fig. 1) indicates tholeiitic to transitional mid-ocean ridge composition. This thick pile of basalts includes lower pillow basalt (Fig. 2), and upper columnar basalts (Fig. 3) recording transition from subaqueous to subaerial flows. The basalts are preserved as slivers within fault blocks between the Chilenia and Cuyania terranes, and were erupted during rifting of these two terranes during the Late Ordovician [1]. The basalts of Sierra del Tigre retain a partial thickness of about 2 km, and comparable basalts from Río Jáchal south to Farallón are spread over a width of 15 km and length of 900 km in the Argentine Precordillera (Fig. 1). Their apparent volume was thus in excess of 27,000 km3: perhaps much greater considering structural compression of their current outcrop.

Geological age of Sierra del Tigre and other comparable basalts of the Argentine Precordillera is best constrained by high resolution graptolite and conodont biostratigraphy of the enclosing Alcaparrosa and Yerba Loca Formations between the late Katian [2] and Hirnantian [3], or about 445 Ma [4]. An old K-Ar date for pillow basalts in the Alcaparrosa Formation of 416 ± 10 Ma is a less reliable constraint [5]: a new program of radiometric dating of these rocks is needed.

Late Katian to early Hirnantian age of the Sierra del Tigre and comparable basalts makes them prime candidates for flood basalts predicted [6] to explain the Late Ordovician mass extinction [7], one of the big five mass extinctions of all time [8].

This prediction [6] was based on abrupt negative carbon isotopic excursions worldwide attributed to atmospheric pollution with massive amounts of isotopically light CH4 from thermogenic cracking of coal or other organic matter around the feeder dikes of flood basalts, as best understood for the Late Permian mass extinction [9]. Alternative hypotheses of Late Ordovician mass extinction include Hirnantian cooling [8], but the first wave of the extinction is before glacial onset indicated by positive marine carbon isotopic values and glacial facies correlations [10]. Another plausible culprit for Late Ordovician mass extinction is oceanic anoxia [8], indicated by black shales with reduced iron, high molybdenum, abundant pyrite framboids, and low sulfide sulfur isotopic values, but these also postdate the mass extinction [11], and could be a consequence of mass mortality [7]. There is no clear evidence of Hirnantian extraterrestrial impact as an extinction mechanism, even in sections where geochemical and petrographic evidence of impact has been sought [12]. Swarms of meteorites in Middle Ordovician (Dapingian) limestones in Sweden are not associated with mass extinction [13]. Thus basalts of Sierra del Tigre in Argentina as a smoking gun of the Hirnantian mass extinction remains a viable hypothesis which deserves further testing.


[1] González-Menéndez, L., Gallastegui, G., Cuesta, A., Heredia, N., & Rubio-Ordoñez, A. Petrogenesis of Early Paleozoic basalts and gabbros in the western Cuyana terrane: constraints on the tectonic setting of the southwestern Gondwana margin (Sierra del Tigre, Andean Argentine Precordillera). Gondwana Res. 24, 359-376 (2013).

[2] Ortega, G., Albanesi, G.L., Banchig, A.L., and Peralta, G.L. High resolution conodont-graptolite biostratigraphy in the Middle-Upper Ordovician of the Sierra de La Invernada Formation (Central Precordillera, Argentina). Geologica Acta 6, 161-180 (2008).

[3] Brussa, E., Mitchell, C. E. & Astini, R. Ashgillian (Hirnantian?) graptolites from the western boundary of the Argentine Precordillera. In: Quo vadis Ordovician?, Short Papers 8th International Symposium on Ordovician System, Kraft , P. & Fatka, O. (Eds.), Acta Univ. Carolinae Geol. 43(1/2), 199–202 (1999).

[4] Gradstein, F.M., Ogg, J.G., Schmitz, M.D., & Ogg, G.M. 2012. The Geologic Time Scale 2012. Amsterdam, Elsevier (2012).

[5] Vilas, J.F., & Valencio, D.A. Palaeomagnetism and K-Ar age of the Upper Ordovician Alcaparrosa Formation, Argentina. Geophys. J. R. Astr. Soc. 55,143-154 (1978).

[6] Lefebvre, V., Servais, T., François, L., and Averbuch, O. Did a Katian large igneous province trigger the Late Ordovician glaciation? A hypothesis tested with a carbon cycle model. Palaeogeogr. Palaeoclim. Palaeoec. 296: 310-319 (2010).

[7] Harper, D.A.T., Hammarlund, E.U., & Rasmussen, C.M.Ø. 2014. End Ordovician extinctions: a coincidence of causes. Gondwana Res. 25, 1294-1307 (2014).

[8] Retallack, G.J. 2015. Late Ordovician glaciation initiated by early land plant evolution, and punctuated by greenhouse mass-extinctions. J. Geol. in press (2015).

[9] Retallack, G.J., and Jahren, A.H. 2008. Methane release from igneous intrusion of coal during Late Permian extinction events. J. Geol. 116: 1-20.

[10] Ghienne, J.-F., Desrochers, A., Vandenbroucke, T.R.A., Achab, A., Asselin, E., Dabard, M.-P., Farley, C., Loi, A., Paris, F., Wickson, S., & Veizer, J. A Cenozoic-style scenario for the end-Ordovician glaciation. Nature Communications 5489, 1-9: DOI: 10.1038/ncomms5485 (2014).

[11] Armstrong, H.A., Abbott, G.D., Turner, B.R, Makhlouf, I.M., Muhammad, A.B., Pedentchouk, N., & Peters, H. Black shale deposition in an Upper Ordovician-Silurian permanently stratified periglacial basin, southern Jordan. Palaeogeogr. Palaeoclim. Palaeoecol 273, 368-377 (2009).

[12] Wang, K., Chatterton, B.D.E., Attrep, M., & Orth, C.J. Late Ordovician mass extinction in the Selwyn Basin, northwestern Canada: geochemical, sedimentological and paleontological evidence. Canad. J. Earth Sci. 30, 1870-1880 (1993).

[13] Schmitz, B., Harper, D.A.T., Peuckner-Ehrenbrink, B., Stouge, S., Alwmark, C., Cronholm, A., Bergström, S.M., Tassinari, M., & Wang, X. Asteroid breakup linked to the Great Ordovician Biodiversification Event. Nature Geosci. 1, 49-53 (2008).

Fig. 1. Geological location (a), map (b) and cross section (c) of the basalts of Sierra del Tigre, Argentine Precordillera [1].

Fig. 2. Pillow basalt of the lower basalts of Sierra del Tigre, 8 km east of Rodeo, Province San Juan, Argentina (site 6 of Fig. 1 at S30.20495o W69.06524o). Bedding is steeply dipping to the left: hammer for scale.

Fig. 3. Columnar basalt of the upper basalts of Sierra del Tigre 8 km east of Rodeo, Province San Juan, Argentina (site 6 of Fig. 1 at S30.20495o W69.06524o). Bedding is steeply dipping to the right. Susanna de la Puente for scale.