What Exactly Are The Back Arc Troughs?

The back arc or miogeosynclinal troughs are asymmetrical shelves whose sediments thicken toward the arc. This type of basin can also be regarded as an elongated epicratonic embayment and is more or less synonymous with Klemme's extra continental downwarp. The deposits of these basins are largely shallow marine shales, carbonates (often reefal), and mature tidal shelf sands, with perhaps a feather edge of non marine sediments. Between major subductive phases the extensive source rocks may be deposited along the basin's seaward margin. Orogenic movements leave these back arc basins terminated on their outer side by a thrust belt. The Arabian Gulf, Sumatra and western Canada illustrate this type of basin.

Back arc basins have a good potential for favorable source rock sedimentation. An extensive marine shelf can have clay blankets deposited during marine transgression. When these clay blankets coincide with uplift of the arc to form a barrier, restricted anaerobic conditions may occur because of poor circulation. The cretaceous shales that were deposited in the great seaway extended from the Arctic to the Gulf of Mexico in the back arc basins of the Rocky Mountains illustrate this point. Traps in the back arc basins are numerous and varied. With this combination of favorable reservoir rocks, source rocks and trap diversity, it is not surprising that such basins are commonly major hydrocarbon provinces.