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A Comprehensive Structural Analysis of Sand Injectites within the Miocene Santa Cruz Mudstone

Stephen Hartwell
University of California Santa Cruz, Department of Earth Sciences, Santa Cruz, California
[email protected]

Late Miocene clastic intrusions (injectites), exposed in northern Santa Cruz county, represent one of the largest subaerial exposed injectite complexes in the world. Clastic intrusions have been recently shown to be important reservoirs for hydrocarbons. Additionally, clastic injection complexes can link formerly discrete reservoirs, allowing hydrocarbons to migrate to higher stratigraphic layers, or even provide a conduit to the surface. Better understanding the structure and evolution of clastic intrusions will aide in the search for hydrocarbons in areas such as the North Sea. The Santa Cruz injectite complex is exposed for roughly 7 kilometers along the coastline, with the northwestern-most exposure just south of Davenport. The largest intrusions occur in the northern section near Yellow Bank creek, where a single intrusion measures more than 150 meters across. Sills are less common then dykes, mostly occurring adjacent to dyke clusters in the central and northern portion of the complex. Sills are typically less than a meter in thickness, extending over a horizontal distance of nearly 200 meters, and commonly vary from horizontal to inclinations of roughly 30˚. Locally, sills merge into dykes with no obvious contact, suggesting that the two are coeval. It is likely that dykes originated at the lower contact of the Santa Cruz mudstone, with sills occurring nearer to the surface where lithostatic pressure is smaller. The relationship between sills and larger injectite structures suggests that horizontal compression around injectites due to dilation during injection may also be a factor.


AAPG Search and Discovery Article #90094 © 2009 AAPG Foundation Grants in Aid