DETRITAL ILLITE CRYSTALS IDENTIFIED FROM CRYSTALLITE THICKNESS MEASUREMENTS IN SILICICLASTIC SEDIMENTS

Abstract

Illite crystals in siliciclastic sediments are heterogeneous assemblages of detrital material coming from various source rocks and, at paleotemperatures > 70 degrees C, of superimposed diagenetic modification in the parent sediment. We distinguished the relative proportions of 2M(t) detrital illite and possible diagenetic 1M(d) + 1M illite by a combined analysis of crystal-size distribution and illite polytype quantification. We found that the proportions of 1M(d) + 1M and 2M(1) illite could be determined from crystallite thickness measurements (BWA method, using the MudMaster program) by unmixing measured crystallite thickness distributions using theoretical and calculated log-normal and/or asymptotic distributions. The end-member components that we used to unmix the measured distributions were three asymptotic-shaped distributions (assumed to be the diagenetic component of the mixture, the 1M(d) + 1M polytypes) calculated using the Galoper program (Phase A was simulated using 500 crystals per cycle of nucleation and growth, Phase B = 333/cycle, and Phase C = 250/cycle), and one theoretical log-normal distribution (Phase D, assumed to approximate the detrital 2M(1) component of the mixture). In addition, quantitative polytype analysis was carried out using the RockJock software for comparison. The two techniques gave comparable results (r(2) = 0.93), which indicates that the unmixing method permits one to calculate the proportion of illite polytypes and, therefore, the proportion of 2M(1) detrital illite, from crystallite thickness measurements. The overall illite crystallite thicknesses in the samples were found to be a function of the relative proportions of thick 2M(1) and thin 1M(d) + 1M illite. The percentage of illite layers in I-S mixed layers correlates with the mean crystallite thickness of the 1M(d) + 1M polytypes, indicating that these polytypes, rather than the 2M(1) polytype, participate in I-S mixed layering.