Diagenesis and Vein Formation in the Miocene Monterey Formation
Veins are ubiquitous but the timing of vein formation, i.e. the opening of fractures and subsequent or concurrent precipitation of mineral cement, is usually not constrained beyond crude categories such as 'early diagenetic' and 'tectonic'. It is also frequently assumed that fractures form preferred pathways for fluid flow thus representing a fairly open system of geochemical mass transfer on a formation scale.

The Miocene Monterey Formation of coastal California is a sequence of organic-rich siliceous mudstone that contains varying proportions of authigenic dolostone, phosphate minerals, and volcanic ash. The principal rock-forming phases of mudstone--biogenic silica, smectitic clay, authigenic dolomite, and organic matter--become increasingly metastable with increasing burial, leading to a complex sequence of diagenetic alteration. Biogenic silica, deposited as metastable opal-A, undergoes a diagenetic alteration sequence to metastable opal-CT and to stable quartz. Authigenic dolomite is initially precipitated as non-stoichiometric and poorly ordered 'proto-dolomite' that becomes more stoichiometric and ordered during subsequent steps of dissolution and re-precipition. Mixed-layer smectite-illite clay increases systematically in illite component. Organic matter undergoes diagenesis and catagenesis, with catagenesis beginning at burial temperatures as low as 60°C.

The formation contains multiple generations of veins (Figure 1). Because of the short and simple burial history of the formation and yet its complex diagenetic history consisting of distinct diagenetic stages, veins in the Monterey Formation are likely to be good candidates to study the interaction between burial diagenesis and vein formation, including the timing of vein formation with respect to the burial cycle and the scale of mass transfer. Based on crosscutting relations observed in outcrop and thin-section, we derived a relative sequence of fracture opening and cementation, consisting of at least four stages with dominant cements of dolomicrite, chalcedonic quartz, baroque dolomite and/or calcite, and blocky quartz, respectively. Each stage contained repeated events of fracture opening and sealing.

This relative sequence of vein formation was tied to the host rock diagenetic alteration based on textural criteria. An example of such textural evidence is shown in the photomicrographs of Figure 2 that depict the incomplete transition of opal-CT (pseudo-isotropic in cross-polarized light) to quartz in the host rock. Quartz in the host rock is optically indistinguishable from quartz in the quartz veins. Also noticeable is the wider fracture opening in the opal-CT-rich layers. Based on these textural observations we deduced that quartz vein formation coincided in this case with the opal-CT to quartz transition in the host rock.

Figure 1: Calcite veins in siliceous mudstone (porcelanite) of the Miocene Monterey Formation, Santa Barbara, California. Note that veins are largely confined to the porcelanite layers, contiuing as normal faults in more clay- and organic-rich shale layers.
Figure 2: Quartz veins in siliceous dolostone, Monterey Formation, with incomplete opal-CT to quartz transformation. left: plain-polarized light, right: cross-polarized light.

Using the stable isotopic composition of vein cement, fluid inclusion thermometry, and known transition temperatures of silica phases we correlated the stages of vein formation with the regional burial diagenetic evolution of the formation and, assuming likely geothermal gradients, with the burial curve. The inferred timing relationships among burial history, diagenetic alteration, vein formation, fluid expulsion, and tectonic deformation are shown in Figure 3.

Figure 3: Timing relationships among burial history, diagenetic alteration, vein formation, fluid expulsion, and tectonic deformation. After Eichhubl and Boles, 1998.

Based on the observed correlation between host rock diagenesis and vein formation we concluded that cementation of these veins closely followed the regional diagenetic evolution at this particular stratigraphic interval, indicative of a fairly closed system of geochemical mass transfer. The same correspondence was not observed along cemented faults that cut across larger stratigraphic intervals and that reflect systems that are open with respect to geochemical mass transfer on an intra-formational scale. We also suggested that the repeated cycles of fracture opening coinciding with distinct diagenetic stages reflect a causative link between diagenesis and fracture formation.

Publications

Eichhubl, P., and Behl, R.J., 1998, Diagenesis, deformation, and fluid flow in the Miocene Monterey Formation of coastal California, in Eichhubl. P., ed., Diagenesis, deformation, and fluid flow in the Miocene Monterey Formation of coastal California: SEPM Pac. Sec. Spec. Publ. vol. 83, p. 5-13. [html full text]

Eichhubl, P., and Boles, J. R., 1998, Vein formation in relation to burial diagenesis in the Miocene Monterey Formation, Arroyo Burro Beach, Santa Barbara, California, in Eichhubl. P., ed., Diagenesis, deformation, and fluid flow in the Miocene Monterey Formation of coastal California: SEPM Pac. Sec. Spec. Publ. vol. 83, p. 15-36. [html abstract]

Eichhubl, P., and Behl, R. J., 1998, Diagenesis, deformation, and fluid flow in the Miocene Monterey Formation of coastal California: AAPG Bulletin, vol. 82, no. 5A, p. 846. [html text]

Eichhubl, P., and Boles, J.R., 1997, Fracture Formation in Relation to Burial Diagenesis, Miocene Monterey Formation, Coastal California: AAPG Annual Convention, Official Program, vol. 6, p. A32. [html text]

last update: 10/25/00

© P. Eichhubl, 2000

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