Stratigraphy of Kenai Group, Cook Inlet, Alaska, and Application of Ecological Shift Plot

V. D. Wiggins, James M. Hill

Regional well-to-well correlations within the five nonmarine formations of the Kenai Group in Cook Inlet, Alaska, are poor at best. The erratic nature of these fluvial deposits, indistinct heavy mineral zonations, and long-ranging palynomorphs have made correlations difficult. Correlations are even more difficult along the basin margin where increased paleotectonic uplift and subsidence have affected deposition. These changes in depositional environments strongly affect electric log character, enough to give the impression that we are correlating like depositional cycles instead of time-stratigraphic units. This leads to incorrect paleodepositional and paleotectonic reconstructions. A new statistical technique called an Ecological Shift Plot was used to help solve these c rrelation problems. Standard palynological slides are prepared from composite ditch samples every 90 ft. The slides are examined, and spores and pollen characteristic of warm and cold climatic extremes within the total flora are identified and counted. A ratio of specific warm and cool spore-pollen taxa is calculated, and the value for each 90-ft interval is plotted on semilog paper. On a completed profile for the Kenai Group, the two upper formations, the Sterling and Beluga, shift rapidly between warm and cool cycles. However, below the unconformity at the base of the Beluga, a strong, distinct shift to warm climatic conditions occurred during deposition of the upper Tyonek Formation. A strong, distinct cool shift then occurs within the lower Tyonek Formation and persists down through he Hemlock Formation. This cool shift occurs below a local unconformity which is informally called the Middle Ground Shoal unconformity. Below the base of the Hemlock, the flora again shift to a warm cycle within the West Foreland Formation. Therefore, in the upper Cook Inlet, the primary oil reservoir section is a distinct cool pulse bracketed between two distinct warm cycles. This distinct cool pulse, although somewhat facies controlled, has been identified throughout the basin and has helped with regional basin correlations.

Although this technique has so far only been used in Alaskan Tertiary sequences, it may aid correlations in other nonmarine sequences where plant spores and pollen are abundant.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.