Lower Eocene carbonate cemented chimneys (Varna, NE Bulgaria): Formation mechanisms and the (a)biological mediation of chimney growth?

Abstract

In the area of Pobiti Kamani (Varna, northeast Bulgaria), massive carbonate cemented columns (“chimneys”, up to 1.5 m diameter and 8 m high) and horizontal interbeds (≤ 1.5 m thickness) occur in dispersed outcrops over an area of 70 km 2 within loose Lower Eocene sands. Field observations and petrographical and stable isotope geochemical characterisation of four studied locations reveal a relationship between these structures and processes of ancient hydrocarbon seepage. Column and interbed structures both consist of similar well-sorted silt- to sand-sized nummulitic host sediments, predominantly cemented by early diagenetic, low-magnesium calcite. Filamentous textures, about 10 μm in diameter and 80–650 μm long, are only locally detected within interparticle calcite cement of columns. Column samples from two sites reveal a similar, linear and inverse covariant trend of δ 13C– δ 18O values, which was interpreted as a mixing trend between two end member fluid/precipitation conditions, i.e. (1) a methane- and/or higher hydrocarbon-derived carbon member characterised by δ 13C values as low as − 43‰ and marine controlled precipitation conditions with δ 18O of − 1 ± 0.5‰ V-PDB and (2) a member with less contribution of methane which was mixed most likely with less depleted carbon sources explaining δ 13C values ranging up to − 8‰ V-PDB. The corresponding, depleted δ 18O values, with many samples clustering around − 8‰ V-PDB, are interpreted in terms of precipitation at elevated temperatures. This suggests the venting system was not a true “cold” seep, sensu stricto. Furthermore, column cross-transects often document an internal pattern consisting of (concentric) zones with distinct isotopic signatures, which vary between the two end members. The mixing and internal pattern of column isotopic data, together with petrographical observations, are qualitatively interpreted as evidence of alternating precipitation conditions, controlled by varying seepage rates of a single fluid source at depth, during build-up of individual chimney pipes near the sediment surface. Based on several field observations, migration of the hydrocarbon-charged fluids in Lower Eocene times was possibly channelled along NE oriented faults. Isotopic signatures of calcite cemented horizontal interbeds, with depleted δ 18O ratios as low as − 8.88‰ V-PDB and variable δ 13C (− 1‰ to − 16‰, mainly around − 5‰ to − 7‰) suggest that ascending fluids contributed to their cementation or resetted the calcite cement isotopic signature, predominantly during periods of active seepage of warmer fluids. Only few petrographical (and preliminary lipid-biomarker) evidence has been found, pointing to the presence or possibly former activity of microbiota, involved in carbon cycling and calcite precipitation, typical of cold seep settings. This may result from diagenetic alteration of organic components. However, considering the processes of chimney formation, a cementation process, governed by the inorganic oxidation of hydrocarbons in which interstitial oxygen is rapidly consumed without bacterial mediation, is considered.