Carbon, Oxygen and Strontium Isotopic Composition Of Diagenetic Calcite And Siderite From The Upper Cretaceous Cardium Formation Of Western Alberta

Abstract

<p>The Upper Cretaceous (Turonian) Cardium Formation of western Alberta consists of a sequence of interbedded mudstones, sandstones and some conglomerates. Siderite is a common mineral constituent in the Cardium Formation and occurs as an early pore-filling cementing phase in marine conglomerates (Carrot Creek) and as concretions in marine mudstones and brackish/continental (Kakwa) sediments. Diagenetically later calcite cements are also common in the conglomerate and sandstone units. Petrographic investigation of the diagenetic mineral phases defines the sequence of mineral precipitation to be pyrite, siderite, calcite, quartz, kaolinite, fracture-filling calcite. The siderites precipitated below the sediment seawater interface under reducing conditions in porewaters depleted in sulphate via sulphate reduction and sulphide precipitation. The reduction of organic rich matter and Fe³⁺ by bacterial processes increased the DIC and Fe/Ca ratio of the porewaters so that siderite precipitated preferentially over calcite. Sedimentary siderites have a high clay content and an analytical procedure was established to prevent contamination from the interlocked, highly radiogenic clays Sr. Marine siderite concretions are distinguishable from their brackish/continental counterparts by their Sr and O isotopic composition. Brackish/continental siderites have lighter oxygen isotopic values and a more radiogenic strontium composition compared to the marine equivalents. Sr and O isotopic composition of the calcites reveals that they precipitated at a later stage, following meteoric water encroachment into the basin.</p>