Phreatomagmatic eruption during the buildup of a Triassic carbonate platform (Oman Exotics): eruptive style, associated deformations, and implications on CO2 release by volcanism Christophe Basile, François Chauvet To cite this version: Christophe Basile, François Chauvet. Phreatomagmatic eruption during the buildup of a Triassic carbonate platform (Oman Exotics): eruptive style, associated deformations, and implications on CO2 release by volcanism. 2006. hal-00116078 HAL Id: hal-00116078 https://hal.archives-ouvertes.fr/hal-00116078 Preprint submitted on 24 Nov 2006 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Phreatomagmatic eruption during the buildup of a Triassic carbonate platform (Oman Exotics): eruptive style, associated deformations, and implications on CO2 release by volcanism Christophe Basile (corresponding author), François Chauvet Laboratoire de Géodynamique des Chaînes Alpines, CNRS UMR 5025 Observatoire des Sciences de l’Univers de Grenoble, Université Joseph Fourier Maison des Géosciences, 1381 rue de la Piscine 38400 St Martin d’Hères, France tel. 334 765 140 69; fax 334 765 140 58 email [email protected]; [email protected] Abstract Oman exotics represent remnants of a Triassic carbonate platform (Misfah Formation). Within these carbonates, and coeval with the sedimentation, several basaltic magmatic events occurred mainly as intrusions, also as lava flows and projections. We describe one of these events, that produced a phreatomagmatic eruption along a volcanic fissure. The initial ascent of magma probably occurred along on a normal fault related to gravity-driven sliding of the carbonates towards the platform edge. Magma first emplaced in a saucer-shaped sill few tens of meters below the surface. This intrusion provided a decollement layer, that may have speed up the gravity-driven sliding, opening fractures that brought sea-water in contact with the magma, hence triggering the phreatomagmatic eruption. Eruption was followed by the collapse of the limestones in a megabreccia infilling the eruptive line and prohibiting further contact between sea water and magma. The main magma volume emplaced at depth in two superposed magma chambers that replaced the host sediments and uplifted the overlying eruptive line. These magma chambers fossilized the substratum of the carbonate platform, that consists in uplifted sediments from the distal Hawasina basin. Replacing limestones by magma chambers may have release huge volumes of Carbon dioxide, estimated to be two to three hundred times higher than CO2 release by volcanic gases. CO2 release by decarbonating sediments may be an important mechanism to explain climatic changes associated to some large igneous provinces such as Siberia, Central Atlantic Magmatic Province, or Karoo, where very large magmatic volumes where intruded in sedimentary basins. Keywords Phreatomagmatic eruptions; sills; magma chambers; megabreccia; carbonate platforms; Carbon dioxide; INTRODUCTION with carbonate platform build-up may GEOLOGICAL SETTING Carbonate platforms commonly strongly, but locally, influence the As part of the southern margin of developped above or on the side of sedimentation. On the other hand, one the Neo-Tethys ocean, the Oman marine volcanos, either in intraplate can expect that sediment-magma continental margin formed during setting (e.g. Hess, 1946; Hamilton, interaction may have some Permian-Triassic times (Béchennec, 1956; Matthews et al., 1974; Schlager, implications on gas-releases during 1988; Robertson and Searle, 1990; 1981; Buigues et al., 1992; Premoli volcanic eruptions, which are Sengör et al., 1993). Palinspatic Silva, Haggerty, Rack et al., 1993; suspected to trigger climatic changes reconstructions suggest the Sager, Winterer, Firth et al., 1993) or (e.g. Wignall, 2001). As carbonates development of contrasted in subduction-related volcanic arcs represent the main reservoir in the sedimentary environments since (e.g. Fulthorpe and Schlanger, 1989; Carbon atmospheric cycle, their Middle Permian times, with a Larue et al., 1991; Watkins, 1993; interaction with magmas may imply continental platform (Saiq Formation: Soja, 1996). However, in most studied significant modifications of Glennie et al., 1974), a continental cases, the carbonate platform builded atmospheric release of CO2 by slope (Sumeini Formation: Glennie et on an inactive volcano. There is only volcanism. al., 1974), and basinal environments sparse observations of interactions In this paper we describe how a (Hawasina Formations: Glennie et al., between volcanism and carbonate volcanic eruption interacted with the 1974; Béchennec, 1988). All these platform sedimentation, mainly formation of a Triassic carbonate sedimentary units were thrusted on the restricted to interstratification of platform in Oman, emphasizing on arabian platform during the upper sediments and lava flows or volcanic small- and large-scale deformations of Cretaceous obduction of the Sumail projections (e.g. Buigues et al., 1992; lithified and unlithified sediments, ophiolites (Béchennec et al., 1988) Soja, 1996; Beltramo, 2003) or magmatic intrusions, deposition of (Fig. 1). It is noteworthy that the exceptionally to shallow-marine volcano-sedimentary formations, paleopositions of the sedimentary eruption through a carbonate platform consequences on local subsidence and units on the continental margin were (MIT guyot: Shipboard Scientific subsequent carbonate sedimentation, mainly derived from their positions in Party, 1993; Martin et al., 2004). It and a rough estimate of CO2 release the tectonic pile, assuming an outward seems obvious that volcanism coeval from the sediments. thrust sequence (Glennie et al., 1973, 1 OBSERVATIONS Stratigraphy The studied volcano-sedimentary section belongs to the lowest part of Misfah Formation, north-east of Jabal Misfah, near the village of Subayb. It may belong to the Subayb Formation as defined by Pillevuit (1993), but we do not support this stratigraphic terminology as both published reference sections appeared to be limited upward by tectonic contacts. The volcanic and sedimentary log described here (Fig. 2) synthesizes observations from the northeastern part of Jabal Misfah, once removed the numerous and more recent magmatic intrusives and fault offsets. However, it is important to note that while carbonates have and important lateral extension, effusive as intrusive magmatic formations occur only locally. Figure 1: Location of the studied area (Jabal Misfah, boxed) in a simplified Above submarine basaltic lava geological map of central and eastern Oman Mountains (modified after flows (pillow lavas and Béchennec et al., 1990). Inset: simplified geological map of northern Oman, hyaloclastites), the sedimentary modified after Glennie et al., 1974. section begins by few beds of volcanic gravels in a carbonate matrix. Then a 1974; Béchennec, 1988; Robertson slopes of Jabal Misfah, and comprises first carbonate unit develops with from and Searle, 1990). There is an ongoing from bottom to top (Fig. 2): bottom to top ten meters of decimeter- debate on the nature of the basement (i) A volcanic unit, made of thick beds of yellow marly limestones, below the Hawasina basin, supposed massive pillow lavas basalts, forty meters of wavy-bedding marly to be either oceanic (Glennie et al., hyaloclastites and tuffites, and dated limestones, two meters of marls, and 1973, 1974; Stampfli et al., 1991; Ladinian-Carnian from foraminifera fifteen meters of thirty centimeters- Pillevuit et al., 1997), continental found in few intercalated limestones thick grey limestones strata. These (Béchennec, 1988; Béchennec et al., (Pillevuit, 1993). This unit thrusts the carbonates are locally truncated by a 1990, 1991) or intermediate (Graham, Late Permian to Liasic bathyal twelve meters-thick volcano- 1980; Searle and Graham, 1982). As a sediments of Al Jil (limestones and sedimentary unit including from consequence, the Permian (e.g. cherts) and Matbat (limestones, bottom to top tuffites, volcanic Pillevuit et al., 1997) or Triassic (e.g. sandstones and siltstones) Formations breccia, sub-marine lava flows and Béchennec, 1988) age for the onset of (Beurrier et al., 1986). peperites. Above begins a second the oceanic accretion is also a matter (ii) The Misfah Formation, made carbonate unit, approximately one of debate. of thinly bedded marly limestones at hundred and fifty meters-thick, made During Triassic times, a carbonate the base (Subayb Formation, Pillevuit, of thirty centimeters-thick beds of platform developped within the 1993), overlain by thick and massive black limestones with few white Hawasina basin. Remnants of this platform limestones. These two stromatolithic layers. Two thin platform were named Oman Exotics members were dated respectively volcano-sedimentary layers are by Glennie et al. (1974), and referred Ladinian-Carnian, and Ladinian- intercalated within this carbonate unit. to the Kwar Group by Béchennec Carnian to Rhaetian by Pillevuit They consist mainly in tuffites, with