AAPG/GSTT HEDBERG CONFERENCE

“Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems”

June 4-7, 2006 – Port of Spain, Trinidad & Tobago

Turtles structures created by extension in a thin detachment setting

Joshua Creviere Turner

BP, Houston, Texas

Turtle structures are generally regarded to be the result of passive or extensional collapse of underlying salt bodies.  However subsurface examples, in the Columbus basin (Trinidad), onshore south Texas (USA), and offshore Golden Lane (Mexico), exhibit four-way inversion structures formed with little or no salt at the detachment level, and look at least superficially like turtle structures.  Furthermore many anticlines in the Shelf region of the Northern Gulf of Mexico are possibly more kinematically akin to extensional turtles than traditional turtle structures.

I propose the term extensional turtles to denote a class of turtle structures at the opposite end of the spectrum from traditional turtles where a mock turtle lies between.  Extensional turtles are generated purely by extension, unlike mock turtles.  Even so, they commonly have some mobile-rock withdrawal, as they are always associated with a very weak detachment.  The distinguishing feature of such structures is that they are bound by a kinematically linked set of regional and counter-regional large-offset listric normal faults.  Local complications may occur where younger regional faults repeatedly cut the counter-regional fault, resulting in the counter-regional fault becoming reset down-dip.  The counter-regional fault may then remain active, resulting in multiple turtle anticlines.  Counter-regional rollover followed by regional rollover on opposing arcuate listric faults produces four-way plunging turtles.

Offshore Louisiana has numerous turtle structures, extensional faults and salt diapirs.  I suggest that many of these turtles bear significant likeness to extensional turtles, and it is likely that a continuum of structural styles exist between true turtles, mock turtles and extensional turtles.