Full citation: 

Carbonate concretions in clastic rocks and their septarian fracture fills act as ‘time capsules’, capturing chemical and biological diagenetic processes during diagenesis. However, the formation of concretions and septarian fractures remains poorly understood as although concretions are a common occurrence in clastic rocks throughout the geological record, they are rarely documented in recent shallow-burial environments. Consequently, the depth and temperature at which various processes occur are often unconstrained. Carbonate clumped isotopes have recently successfully been applied to concretions and some fracture fills. However, too few data exists to determine the boundary conditions for the formation of concretions and septarian fractures. Here, we present carbonate clumped isotope results of fracture fills from eight different concretions from various locations, including multiple phases of fill in 4 concretions. Our results suggest that they precipitated over a range of temperatures (22°C - 85°C) from δ18Oporewater values between -12‰ to 3‰ and within different δ13Ccarbonate zones. The majority of fills precipitate at lower (<50°C) temperatures, although the fluids are not always meteoric. For 3 concretions containing fractures with multiple phases, the δ18Oporewater becomes progressively heavier with each later phase and increasing temperature. The one exception to this is in the Barton Clay Formation (UK) where the fractures must have been continuingly infilled during exhumation as the latest cement phase is the coolest with a δ18Oporewater more depleted than the earliest phase. Therefore, concretion growth must usually initiate early on (<~1 km burial), and subsequent fracturing is also usually early. However, the fracture infilling can occur over a range of depths and can record the diagenetic history of a formation. We gratefully acknowledge a BP and EPSRC Case Studentship for funding this project, and the Natural History Museum London for providing samples.

Publication date: 
Thursday, April 10, 2014