Influence of Depositional Environments on Origin of Secondary Carbonates and Diagenesis of Organic Matter, Tertiary Basinal Units, La Honda Basin, Santa Cruz Mountains, California

David J. Hollander, Robert E. Garrison

A comparative study of authigenic carbonates formed in five organic-rich, deep-water units of the La Honda basin indicates that sedimentation rate and degree of bottom-water oxygenation ultimately controlled the timing, chemical environment, mineralogy, and rate of authigenic carbonate formation. Additionally, sedimentation rate and oxygenation of bottom water governed the sequence of diagenetic chemical environments, the degree of preservation of organic matter, and the source bed potential of the basinal units.

Associated with authigenic carbonates formed in basinal units having high rates of sedimentation (> 100 m/m.y.) and high rates of primary productivity (200-350 g HC/m²/year) (lower Miocene Lambert Shale, middle-upper Miocene Monterey Formation, and upper Miocene Santa Cruz Mudstone) were methanogenic diagenetic environments, rapid growth rates of carbonate concretions (0.3-1.2 mm/10³ years) and beds (1.9 mm/10³ years), low rates of organic matter consumption during authigenic carbonate formation (12.95% TOC loss/100 m change in depth of formation), and a sequence of authigenic phases dominated by iron-rich dolomite and calcite. In contrast, authigenic carbonate minerals formed in basinal units with low rates of sedimentation (< 100 m/m.y.) and lo rates of primary productivity (< 50 g HC/m²/year (middle-upper Eocene Twobar Shale Member and upper Eocene-Oligocene Rices Mudstone Member) had sulfate-reducing diagenetic environments, slow growth rates of carbonate concretions (0.02-0.07 mm/10³ years) and beds (0.303 mm/10³ years), high rates of organic matter consumption during authigenic carbonate formation (22.05% TOC loss/100 m change in depth of formation), and a sequence of authigenic phases dominated by sulfide minerals and iron-poor carbonates.

The conditions necessary for the preservation of organic matter and the development of potentially good source rocks were variable. However, the occurrence of anaerobic bottom waters was the critical element in the development of excellent source rocks (i.e., Twobar Shale Member, Lambert Shale) regardless of the rate of sedimentation or the rate of primary productivity. In contrast, in basinal units characterized by oxygenated bottom waters, high rates of sedimentation, and high rates of primary productivity (Monterey Formation and Santa Cruz Mudstone Member), rapid burial of organic matter promoted highly anaerobic diagenetic environments which decreased the rate of organic matter decomposition, thereby producing units with good potential source rocks.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.