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Hydrothermal Dolomitization in Paleozoic Carbonates - Enhanced Fluid Flow and Foreland Basin Processes*

By

Denis Lavoie¹ and Guoxiang Chi²

Search and Discovery Article #50049 (2007)

Posted August 10, 2007

*Adapted from oral presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007

¹Geological Survey of Canada, Quebec City, QC ([email protected])

²University of Regina, Regina

Abstract 

In North America, world-class reservoirs are hosted by Paleozoic hydrothermal dolomites. These fields are found in successions affected by tectono-magmatic processes linked to foreland basins. In eastern North America, Taconian (Middle-Upper Ordovician) and Acadian (Upper Silurian-Lower Devonian) foreland basins generated tectono-magmatic conditions favourable for the formation of hydrothermal dolomites (HTD).  

The Ordovician HTD cases occur in passive margin to foreland basin carbonates. Hot saline fluids moved along Taconian extensional faults commonly rooted in crystalline basement; these fluids were laterally forced into carbonate units when reaching an effective seal. Coeval K-bentonite deposits testify to major volcanic centres and hence high geothermal gradients critical for enhancing deep-seated fluid convection. The Lower Silurian peritidal ramp facies is dissected by older faults that were reactivated at the late Early Silurian onset of the Acadian foreland basin. The major reef complexes and pinnacles of Upper Silurian to lowermost Devonian were positioned on the paleotopographic highs of tilted extensional tectonic blocks. Lower Devonian outer shelf carbonate facies were deposited in a faulted depositional setting; these faults were in late Early Devonian, reactivated as dextral strike slip. These faults were conduits for episodic high pressure migration of high temperature saline fluids that resulted in local to regional hydrothermal dolomitization. Enhanced fluid convection was generated by high geothermal gradients that resulted from active volcanism episodically recorded from the Early Silurian to the Early Devonian.  

Lower Paleozoic tectono-magmatic events created conditions for the hydrothermal alteration of carbonates. The Ordovician and Devonian reservoirs host economic accumulations of hydrocarbons.

Selected Figures for General Overview of the Paleozoic Paleosouthern Margin of Laurentia

(Facies and faults, volcanism-magmatism, and fluids and HTD in the Taconian (Ordovician) foreland basin and in the Salinic-Acadian (Silurian-Devonian) foreland basin)

The Basics 

• The inception of foreland basin produces an early transtensional to extensional tectonic regime.

• Fluid pressure buildups at deeper level will result in rapid upward, fault-focused movement when the fault is reactivated.

• Upward movement and convection will be enhanced if regional thermal gradients are high.

• When circulating in a limestone succession, the fault-focused, high-energetic pulses of high-temperature fluids will leave a significant imprint (texture and geochemical) – the resulting product is known as hydrothermal dolomite.

The problem: the driving mechanism for significant fluid circulation to produce the high volume of dolomite.

Additional Selected Figures 

Conclusions 

Ordovician HTD bodies are associated with extensional to transtensional faults and interbedded with felsic volcanic ashes resulting from Taconian arc subduction and associated volcanism. 

Lower Silurian, Upper Silurian – lowermost Devonian HTD units are associated with early extensional to late transtensional faults. These HTD(s) are coeval with significant Early Silurian arc subduction, Late Sillurian to Early Devonian slab detachment, all of which resulted in felsic to mafic magmatism and volcanism. 

A link between HTD and extensional-transtensional faulting is accepted by most; however, the possible (critical??) association with magmatism/volcanism for enhanced circulation still remains to be better documented.

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