Distributed fracturing affecting isolated carbonate platforms, the Latemar Platform Natural Laboratory (Dolomites, North Italy)

Abstract

Isolated carbonate platforms are highly heterogeneous bodies and are typically composed of laterally juxtaposed first order domains with different sedimentological composition and organization, i.e. a well-stratified platform interior, a massive margin and a slope with steeply dipping and poorly organized layers. Once subjected to overburden and tectonics forces, directions and magnitudes of principal stresses inside such heterogeneous bodies are expected to deviate significantly from the regional trends potentially resulting in non-obvious fracture patterns. In addition, characteristics such as fracture height and spacing distances are likely to change moving from one domain to the other as well as through time.The Middle Triassic Latemar platform (Dolomites, N Italy) is affected by ubiquitous fractures, typically joints. Orientation and height of more than 1500 fractures from 33 outcrops have been measured and their spatial characteristics were assessed with dedicated software. Fractures are organized in two directional sets, NNW–SSE and ENE–WSW. The orientation of each set is constant throughout the platform, showing that the trajectories of causative stresses were not influenced by the platform architecture. Fracture spacings and heights of each set change moving from one domain to the other. In the well-bedded platform interior, fractures are generally <2 m tall and have spacing distances <1 m controlled by the thickness of the hosting fracture unit and its dominant lithology. Only <50% of the fractures terminates at ±2 cm from the strata boundaries with grainstones–grainstones contacts being the most effective in preventing vertical fracture propagation. In the platform margin, fractures are up to 10 m tall and spacing distances are typically <5 m. In the massive, poorly organized slope fractures are ca. 10 m tall and have spacings of 5–10 m.There are also significant differences in the characteristics of fractures of the two sets. In all platform domains, fractures of NNW–SSE set are less tall and more narrowly spaced than those of the ENE–WSW set. In the platform interior fractures of the first set have a higher tendency to be confined to fracture units than those of the second set. These differences are associated with diagenesis-driven changes in bulk mechanical properties of the platform during its subsidence to maximum burial depth.