Sedimentology and geochemistry of gas hydrate rich sediments from the Oregon margin (Ocean Drilling Program Leg 204)

Abstract

Gas hydrates have been recently recognized as a key factor affecting a number of global processes such as the climatic change, sea floor stability, etc. In this thesis we present the multidisciplinary study of gas hydrate rich sediments recovered during ODP Leg 204. The main objective of this thesis is to study how the textural characteristics of marine sediments can affect the main pathways and intensity of fluid flow and how fluid flow determines the distribution of gas hydrates in the continental margins, as well as the main geochemical processes that occur during early diagenesis.<br/><br/>To reach these objectives, a complete sedimentary and geochemical study of 581 sediment samples from southern Hydrate Ridge was carried out. The methods and techniques that were applied include: complete textural analyses, mineralogy, physical properties and geochemistry.<br/><br/>The southern Hydrate Ridge sediments are mainly made up of four lithofacies defined as: hemipelagites, turbidites, ash layers and debrites. Mass&#8208;transport deposits such as turbidites and debrites are more abundant in Lithostratigraphic Unit III and II, as well as in Lithostratigraphic Unit IA in the slope basin of southern Hydrate Ridge. Some increasing trends with depth can be observed in the smectite content in the clay mineral assemblages. These features suggest that the transport in suspension of fine sediments through the California Current was more effective during the Pliocene and early Pleistocene period. Bedload transport of coarse material from local and distal areas was more effective during the middle Pleistocene and Holocene due to the tectonic reactivation of the southern Hydrate Ridge uplift. During the Pleistocene and owing to the pervasive fluctuation of sealevel, gas hydrate dissociation together with the seismic movements in the Oregon margin seems a plausible triggering mechanism for mass&#8208;movements.<br/><br/>The results presented here confirm that the sedimentation patterns in the Hydrate Ridge region are controlled by climate and tectonic parameters such as the regional intensity of the California Current or the local tectonic movements that lead to the uplift of the Ridge. These parameters mainly control the clay mineral distribution as well as the sedimentary facies that were produced.<br/><br/>The sedimentary fabric of gas hydrate&#8208;rich intervals is disturbed during core recovery due to gas hydrate dissociation. The two main disturbance fabrics generated through this process are mousselike and soupy. The gas hydrate&#8208;rich sediments analyzed for this thesis are coarser grained in respect to the hemipelagite sediments. The coarse&#8208;grained layers such as turbidites and ash layers could act as conduits for fluids in the southern Hydrate Ridge region because of their higher porosity and permeability. In this context, methane&#8208;rich fluids migrate through these layers from deep in the sedimentary sequence and into the gas hydrate stability zone. A number of barium fronts have been identified in southern Hydrate Ridge sediments and interstitial waters. Barite fronts were formed as a result of the barite recycling process during early diagenesis, which is controlled by the availability of methane&#8208;rich fluids, in situ decomposition of organic matter and the sulphate gradient. Modelling of these data shows that these processes were active at southern Hydrate Ridge for a period of up to one thousand years.<br/><br/>A number of geochemical and sedimentological processes are proposed in this thesis as plausible mechanisms to allow the survival of the barite fronts during diagenesis. The sedimentary texture plays an important role in controlling the major fluid flow pathways in the continental margins. The temporal evolution of the fluid flow can be studied in a given area through the distribution of the mineral phases that form during early diagenesis, as well as the interstitial water composition.

<i>EXTRACTE DE TESI:<br/><br/>Aquesta tesi integra els resultats de l'anàlisi sedimentológica i geoquímica de sediment marins rics en hidrats de gas, recuperats durant la campanya "Ocean Drilling Program" Leg 204 en el marge d'Oregon (USA). L'objectiu principal d'aquest estudi és conèixer les característiques sedimentàries que afecten el fluxe de fluids i gasos a través del sediment i com els fluids afecten la distribució d'hidrats de gas en aquesta àrea, així com alguns processos geoquímics que operen durant la diagènesi inicial. Els mètodes i tècniques aplicats inclouen l'anàlisi de sedimentològia, mineralogia, susceptibilitat magnètica i geoquímica.<br/><br/>Els sediments de southern Hydrate Ridge estan formats per 4 litofàcies: hemipelagita, turbidita, cendra volcànica i debrita. La sedimentació està controlada per factors climàtics i tectònics com ara la intensitat del corrent oceànic Californià o moviments tectònics locals. Aquests paràmetres exerceixen un control fonamental en la distribució dels minerals d'argila i de les fàcies sedimentàries en el marge continental. La seva evolució des del Pliocè és discutida en aquesta tesi.<br/><br/>Els sediments analitzats rics en hidrats de gas són més grollers que els sediments hemipelàgics. Els sediments més grollers actuen com a conductes preferents per a la circulació de fluids degut a la seva porositat i permeabilitat. En aquest context, fluids rics en metà migren des dels sediments profunds cap a la zona d'estabilitat dels hidrats de gas, on possibiliten la seva formació.<br/><br/>Diversos fronts de barita han estat identificats en els sediments de southern Hydrate Ridge. Es formen com a resultat del reciclatge de barita durant la diagènesi inicial, controlada per la presència de fluids rics en metà, la degradació de matèria orgànica i la presència de sulfat. La modelització de les dades obtinguda mostra que aquest procés va ser actiu durant un període de >1000 anys. En aquesta tesi, es discuteixen els possibles processos geoquímics i sedimentaris que permetrien la supervivència de la barita durant la diagènesis.<br/><br/>La textura sedimentària juga un paper molt important en el flux de fluids als marges continentals. La seva evolució temporal en una àrea determinada pot ser deduïda estudiant les fases minerals que es formen durant la diagènesi.</i>