Origin of stylolite related fractures in Atoka Bank Carbonates, Eddy County, New Mexico, U.S.A.

Abstract

Stylolite-related fractures are rare diagenetic features (1% in volume) in Atoka (Carboniferous) bank carbonates in Eddy County, New Mexico, U.S.A. The fractures are often wedge- to curvilinear-shaped, vertical to subvertical, and their wider ends terminate at the seams of major stylolites. The associated stylolites are centimeters in size, low to medium amplitude (up to 1.5cm) and some of them are fabric selective forming at matrix-grain and matrix-cement boundaries. Fractures are filled with predominantly saddle dolomite and late calcite cements. Fracture size ranges from microscopic to 1cm, and they have a width less than 1 mm. A close coexistence of fractures with stylolites suggests that fractures have formed synchronously with the stylolites. In Atoka carbonates, stylolites were formed by progressive burial that is evidence by bedding-parallel or subparallel orientation and different mineral associations along stylolites and also at near areas. These minerals are commonly late calcite and saddle dolomite. Soon after initial stylolitization, dissolved material and/or formation water and surface morphology of stylolites caused localized stress differentiation around stylolite. Compaction differences of less soluble parts at the both sides of stylolitic-highs or lows such as column/peak or waveform result in breakage of less soluble parts above or below column/peak or wave-form. Later wedge-shaped fractures are occluded by late cements provided by stylolitization or stylolitic-channels carring cement-generating fluids from an external source such as dewatering of adjacent shales. Stable isotope values of saddle dolomite (δ¹⁸O=−3.5 to −4.0‰ PDB; δ¹³C=+0.04 to +0.67‰ PDB) and late calcite (δ¹⁸O=−5.4‰ PDB; δ¹³C=+0.7‰ PDB) suggest precipitation at paleotemperatures of 52–55°C for dolomite and 42°C for late calcite based on oxygen values. The temperatures correspond to burial depths of approximately 1450–1600m for dolomite and 1000 m for calcite under present-day conditions, assuming a geothermal gradient of 22°C/km and a surface temperature of 20°C (well data).