This paper presents a detailed analysis of microfacies of nine of the 15 hydrocarbon seep carbonates recorded from the Sassenfjorden area, Svalbard. These Late Volgian-latest Ryazanian carbonates have been found in the black shales of the uppermost Slottsmøya Member, Agardhfjellet Formation. The interval is characterised by a low sedimentation rate, which is testified by the presence of phosphatic grains and glaucony. Most of the carbonate formed within the sediment and is represented by two microfacies. Carbonate formation occurred mainly within the more permeable sediment pelletised by deposit feeders with micrite precipitating in the immediate surroundings. A difference in permeability caused localised fluid overpressure and episodes of hydrofracturing of calcareous mudstones, resulting in the localised occurrence of breccias. Surficial seep carbonates represent a variety of the microfacies, consisting of sponge boundstones and bioclastic sediments.
These microfacies are relatively rare and mostly restricted to one or a few seeps and represent a spatial and temporal variation of surficial seep subenvironments, related perhaps to the type of substrate and fluid flow. Most of the seeps were relatively small and of short duration, which resulted from original sediment properties and perhaps from sealing of original fluid pathways by carbonate cementation. Only seep 9, developed in more permeable sediments, was larger and long-lasting. The prolonged duration of the seepage at this locality resulted in accumulation of a thick cover of bioclasts containing abundant microborings. These traces are made exclusively by chemoautotrophs and heterotrophs and indicate likely palaeodepths exceeding the depth of the photic zone during the latest Ryazanian. The stratigraphic distribution of microfacies indicates a change of original sediment from muds to more permeable silts and sands in the Late Ryazanian, which may be attributed to a shallowing episode