Interpretation of oscillatory sedimentary patterns in a 7.5 meter core, Santa Maria Basin offshore, may record signatures coincident with sunspot activity cycles and/or with Milankovitch time frequencies of earth-orbital interactions. The location of the well, Texaco USA: OCS-P0496 #1 (UTM Zone 10; X=688, 455 M; Y=3, 839, 592 M; Lat. 34.406 degrees and Long. 120.563 degrees) from which Monterey Formation core samples were obtained, represents the most north-western well drilled to date in the offshore Santa Maria Basin. The entire Mid-to Upper Miocene Monterey Formation preserved in this well is represented by 466 feet. Depositional patterns were studied from Core #1, (recovery was from 3500 feet to 3522.5 feet), approximately 180 feet below the top of the Monterey Formation as correlated by e-logs.
In this elongate synclinal basin, the Monterey Formation, a heterogenous sequence of highly biogenous sediment was deposited between late early and latest Miocene time. Foraminiferal and palynologlcal faunal and floral assemblages suggest an Upper Miocene, Upper Mohnian age assessment. Foraminiferal communities furthermore suggest a lower bathyal environment for sediments preserved within the cored interval.
Banding patterns of first-order rhythmicity which approximate the appearance of varves, range in thickness from approximately 0.2mm to 4mm. Higher orders of rhythmicity appear to be lacking in this interval. The well-preserved expression of laminations is observed from the repetition of light and dark banding patterns. The lighter intervals result from a combination of dolomite, quartz and argillaceous rich sediments. The darker bands are exclusively kerogen rich and microscopically appear to represent what may be interpreted as bacterial mat accumulations. Deposition of these biogenic sediments was undoubtedly within an anaerobic environment (viz. dissolved oxygen concentrations less than 0.1 ml/L water) as evidence of bioturbation is completely lacking. Organic matter within this core interval is approximately 7-8%, sapropelic kerogen averages 75% and TAI is 2-4%.
The distribution, nature and spatial patterns of these deep basinal sediments was controlled by the interplay of numerous variables, principal among which include diatom and foraminiferal productivity and subsequent dissolution, eustatic sea level changes and tectonism. Such influences on the general sediment package were, in part, a reflection of the broader global paleoclimatic and oceanographic changes. These changes may be the result of interactions of dominant periods of sunspot activity or Hale cycles (viz. respectively, the 11-year solar activity cycle or the 22-year solar magnetic reversal cycle) or of the earths solar orbit; periods which are within the Milankovitch time frequency band of precession (21,000 years), obliquity (41,000 years) or the shorter eccentricity cycles (100,000 and 413,000 years). Detailed spectral microdensitometry on thin-sections and photographs and Fourier analyses of the first-order cycles will hopefully yield insight into forcing by mechanisms other than autocyclic. Such investigations, at the writing of this report, are still in progress. (A complete text is available from the author.)
