MULTIPLE ASH ACCUMULATION EVENTS PRESERVED IN INDIVIDUAL LOWER PALEOZOIC K-BENTONITES: STRATIGRAPHIC IMPLICATIONS

Explosively erupted volcanic ash preserved as bentonites and K-bentonites can serve as ideal regional stratigraphic markers as well as recorders of tectonomagmatic events. However, tephra deposits are often the result of multiple rather than single eruptive episodes. Moreover, individual explosive eruptions can generate secondary co-ignimbrite plumes which can add complexity to the internal stratigraphy of the tephra deposit. Co-ignimbrite ash clouds are entrained from hot, dense pyroclastic flows resulting from collapsed plinian columns, and may originate either close to or at some distance from the vent. They tend to be enriched in fine vitric ash and rise by convection to as much as 30 km. Sustained co-ignimbrite plumes can be responsible for the distribution of ash at distances in excess of 1000 km compared to plinian eruption columns, which tend to distribute clasts on the order of hundreds of km maximum from the vent. However, if the co-ignimbrite (or Phoenix) plume originates above or close to the vent theoretical plinian column models might be used to estimate minimum mass eruption rate and column height. The co-ignimbrite column will achieve the shape of a buoyant plume as it acts to conserve mass during upward acceleration. Estimates of the amount of pyroclastic material entrained in this process are on the order of 35 percent of the total erupted mass so the missing proximal facies may amount to as much as twice the volume of the preserved ash. Microprobe data on biotites from the Caradocian Deicke and Millbrig beds in North America suggest that the Deicke represents a compositionally homogeneous magma and a single eruptive event. However, the Millbrig shows considerable within-bed variability and most likely contains a co-ignimbrite phase. Millbrig biotite crystals range from only slightly different from Deicke to those characteristic of a more highly evolved parental magma. The magnesium number (Mg2/(Mg2+Fe2)) of the biotites, which serves as a differentiation index, suggests that the Millbrig variability is the product of a continuously evolving magmatic source.