Stylolites and stylolite networks as primary controls on the geometry and distribution of carbonate diagenetic alterations

Abstract

There is an ongoing debate on whether stylolites act as barriers or conduits for fluids, or even play no role in terms of fluid transport. This problem can be tackled by examining the spatial and temporal relationships between stylolites and other diagenetic products at multiple scales. Using the well-known Lower Cretaceous Benicàssim case study (Maestrat Basin, E Spain), we provide new field and petrographic observations of how bedding-parallel stylolites can influence different diagenetic processes during the geological evolution of a basin. The results reveal that stylolites can serve as baffles or inhibitors for different carbonate diagenetic reactions, and act as fronts for dolomitization, dolomite recrystallization and calcitization processes. Anastomosing stylolites, which pre-date burial dolomitization, likely acted as a collective baffle for dolomitization fluids in the study area, resulting in stratabound replacement geometries at the metre-to-kilometre scale. The dolomitization front weaves up and down following consecutive anastomosing stylolites, which are typical of mud-dominated facies that characterize limestone-dolostone transition zones. Contrarily, dolostone bodies tend to correspond to grain-dominated facies characterized by parallel (non-anastomosing) stylolites. The same stylolites subsequently acted as fluid flow conduits and barriers again when the burial and stress conditions changed. Stylolites within dolostones close to faults are found corroded and filled with saddle dolomite riming the stylolite pore, and high-temperature blocky calcite cements filling the remaining porosity. The fluids responsible for these reactions were likely released from below at high pressure, causing hydraulic brecciation, and were channelised through stylolites, which acted as fluid conduits. Stylolites are also found acting as baffles for subsequent calcitization reactions and occasionally appear filled with iron oxides released by calcitization. This example demonstrates how the same type of stylolites can act as barriers/inhibitors and/or conduits for different types of diagenetic reactions through time, and how important it is to consider their collective role when they form networks.

Depositional and structural controls on a fault‐related dolostone formation (Maestrat Basin, E Spain)

“…Therefore, bedding‐parallel stylolites pre‐date dolomitisation which is supported by: (1) similarities in stylolite morphologies observed in unaltered limestones and dolostones (Figures 6h and 10d,l) (as also reported by Humphrey et al, 2020 elsewhere in the Maestrat Basin), and (2) stylolites frequently acting as dolomitisation fronts, indicating that, at least in part, stylolitisation occurred before replacement (Figure 9e). A similar relationship between early calcite cements and chemical compaction was observed in the Benicàssim case study (Martín‐Martín et al, 2015; Gomez‐Rivas et al, 2021).…”

“…However, bedding‐parallel stylolites have influenced the geometry of stratabound dolostones on a centimetre‐scale. Dolomitisation fronts follow stylolite traces in stratabound geobodies (Figures 9e and 10l) suggesting that stylolites have acted as baffles to dolomitising fluids, observed elsewhere by Humphrey et al (2019) and Gomez‐Rivas et al (2021).…”

“…e . above the main dolostone body) and limestone stringers within the dolostones feature bedding‐parallel stylolites which are likely to have inhibited fluid flow (Bruna et al, 2019; Gomez‐Rivas et al, 2021; Humphrey et al, 2019; Martín‐Martín et al, 2015). The relationship between different limestone and dolostone geometries relative to the original host limestone lithofacies suggests that depositional heterogeneity affects the type and distribution of geobodies.…”

“…Within this sub‐basin a variety of dolomitisation geometries have been described in the Benicàssim area, where massive dolostone patches close to fault zones laterally evolve to stratabound geobodies (with metre‐scale thicknesses) that extend laterally for several kilometres away from these structures. Work by Gomez‐Rivas et al (2014, 2021), Martín‐Martín et al (2013, 2015, 2018) and Yao et al (2020) established spatial variations in dolostone geometry associated with proximity to feeding faults, overlying sealing units, host rock facies, as well as sub‐seismic structural features such as stylolites and fractures.…”

“…g . Hirani et al, 2018; Gasparrini et al, 2017; Gomez‐Rivas et al, 2014, 2021; Lapponi et al, 2011; Martín‐Martín et al, 2013, 2015, 2018; Nader & Swennen, 2004; Sharp et al, 2010). In some cases, both geometries appear connected forming a continuum ( e .…”