Stratigraphic and sedimentologic analysis of rhythmic physical, biological, and geochemical signatures in two cores of the Mississippian (Osagean) Borden Group, Harrison County, Indiana

Strata of the Borden Group comprise an ancient marine deltaic system which is divided stratigraphically, using traditional methodologies (lithologic, paleontologic, and geophysical analysis), into three distinctive facies (delta platform, delta slope, delta toe). These methods provide information on sedimentologic, paleontologic and mineralogic properties within deltaic settings but do not necessarily enable researchers to recognize or statistically measure the relationships that exist between these properties. Recognition of subtle, locally rhythmic, variations in sedimentologic properties within each facies of the Borden inspired this research, the purpose of which is to determine quantitatively whether or not the attributes of lithology, rock color, bioturbation and chemical composition are intrinsically related and to test whether or not any such relationships are laterally persistent or systematically repetitive within the deltaic system. This demonstrates that each Borden facies is characterized by specific relationships between the above-mentioned attributes, and that these relationships are laterally correlative. Physical attributes of the cores were described visually, digitized using a discretizing program, and then subjected to rigorous statistical analysis.; Chemical and statistical analyses reveal that as grain size decreases rocks are more likely to be either dark gray or green, and as levels of organic carbon increase the measured levels of reactive iron tend to decrease. Lowest levels of organic carbon and highest levels of reactive iron are found in coarser-grained rocks that are either light gray or red. Burrow size is a function of grain size, and bioturbation intensity decreases as grain size increases and rock color becomes lighter gray or red.; Within the Borden delta, specific depositional mechanisms controlled grain size and sedimentation rates, affected bioturbation style and intensity, and limited the availability of organic carbon, which affected the abundance of reactive iron and thereby controlled the development of rock color during diagenesis. Interrelationships of lithology, ichnofacies index, sediment color and levels of TOC are interpreted as products of changing depositional regime, sediment supply and conditions of water-column chemistry within the depositional basin. Stratigraphic rhythmicity, though possibly affected by Milankovitch-scale allocyclic processes, is primarily a result of autocyclic depositional processes, especially including turbidity currents.