TRENDS IN CLAY MINERALOGY ACROSS A TERRESTRIAL SEQUENCE BOUNDARY IN THE CAMPANIAN JUDITH RIVER FORMATION, MONTANA

Trends in the relative abundances of clay minerals were documented across a discontinuity previously interpreted as a sequence boundary in terrestrial strata of the Campanian Judith River Formation of north-central Montana. Prior to this study, the discontinuity had been identified in outcrop and well logs by an abrupt shift from sandstone- to mudstone-dominated facies and a dramatic increase in fossil content, with strata above the discontinuity yielding considerably more vertebrate fossils. A section spanning ~180 m and encompassing the entire Judith River Formation was measured, and samples were collected from 43 beds below and 44 beds above the discontinuity. The clay fraction of each sample was analyzed using X-ray diffraction, and the abundances of the clay minerals were assessed using the normalized relative intensity ratio method. Clay minerals identified included kaolinite, smectite, illite, and Fe-rich chlorite. Focusing first on all lithologies sampled, a statistically significant decrease in the average relative abundance of kaolinite from 39% below to 23% above the discontinuity (p=0.012) and a corresponding increase in smectite from 32% below to 46% above the discontinuity (p=0.023) was detected. To control for potential effects of host lithology, a second analysis of clay content was confined to sandstones. The results again show a significant decrease in the average relative abundance of kaolinite from 74% below to 38% above the discontinuity (p=4.7 x 10^-5) and a significant increase in smectite from 14% below to 47% above the discontinuity (p=9.8 x 10^-4). Illite and chlorite content remain essentially unchanged across the discontinuity. Radioisotopic age data indicate that rates of rock accumulation increase by a factor of four across the discontinuity, and trends in clay mineralogy are interpreted to likely reflect the duration of weathering on the Judith River coastal plain, with the neoformation of kaolinite inhibited in the more rapidly deposited strata overlying the discontinuity. Regardless of the actual cause(s), statistically significant differences in clay content serve to distinguish facies above and below the through-going discontinuity bisecting the Judith River Formation in its type area.