Fabric anisotropy induced by primary depositional variations in the silt: clay ratio in two fine-grained slope fan complexes: Texas Gulf Coast and northern North Sea

High-resolution X-ray texture goniometry (HRXTG) has been used to quantify the alignment (anisotropy) of phyllosilicate minerals in a series of diagenetically altered mudstone samples from the deep subsurface of the Texas Gulf Coast and the northern North Sea. The sampled formations have undergone a simple burial history with no overprint of tectonic deformation that may have influenced the phyllosilicate orientation. Observations have been made in two constrained temperature windows 148–150 °C (North Sea data) and 169–210 °C (Texas Gulf Coast) wherein less than 20% smectite remains in the mixed layer phase illite–smectite. Quantitative X-ray powder diffraction (XRPD) shows quartz (which is dominantly detrital) and phyllosilicate content having an inverse relationship. Quantified illite–smectite fabric intensity (phyllosilicate alignments) ranges from low alignment values of 2.42 m.r.d. (multiples of a random distribution) to high alignment values 6.75 m.r.d. and chlorite + kaolinite ranges from 1.80 to 6.46 m.r.d. We propose that the broad range of phyllosilicate fabric intensity observed reflects the range of quartz/phyllosilicate ratios that are the imprint of depositional setting, bedform truncation, and initial grain size. Detrital quartz acts to disrupt fabric intensity by being non-platy, producing areas sheltered from effective stress (‘pressure shadows’), and acting as a matrix support. There is no evidence that bioturbation is a significant control on phyllosilicate alignment in these quartz/phyllosilicate mixtures. These findings have important implications for the role of grain-scale alignment on seismic anisotropy in mudstone systems. Fabric intensity in post-diagenetic samples can be predicted from (1) quartz content and (2) total clay content where:equation(1)y=−7.4x+64.8y=−7.4x+64.8x is fabric intensity (in m.r.d.) and y is total (wt.%) quartzequation(2)a=4.2b+33.7a=4.2b+33.7and a is total (wt.%) clay content and b is fabric intensity (in m.r.d.).