Carbonate Buildup Development Within a Passive-Margin Setting: Upper Jurassic of Lusitanian Basin, Portugal, and Comparable Carbonates from Eastern North American Seaboard

P. M. Ellis, R. C. L. Wilson

Several carbonate buildups occur within the Upper Jurassic of the Lusitanian basin, Portugal. Their development can be related either to halokinetics or reactivation of basement fault blocks.

In most plases, the buildups contain only minor volumes of recognizable biogenic framework. The basement-controlled buildups, which have shelf profiles, are relatively thin (200-500 m), show great lateral facies variation, and are predominantly micritic with only thin grainstone and packstone sequences. The salt-controlled buildups, which have a ramp or distally steepened ramp profile, are relatively thick (500-1,000 m or more), show little lateral facies variation, and contain thick sequences of grainstones and packstones.

Increased subsidence rates during the early Kimmeridgian resulted in the drowning of many of the buildups, followed by an influx of clastics throughout most of the Lusitanian basin. In the southern, clastic-starved part of the basin, however, a prograding carbonate ramp sequence passively filled the newly deepened basin. This buildup is micrite dominated, containing only minor volumes of recognizable biogenic framework.

Upper Jurassic carbonate buildups are also known from offshore Morocco and beneath the eastern North American continental shelf. A series of prograding clinoforms occurs in the Baltimore Canyon area, for example, which show similar thicknesses and grainstone-dominated composition to the salt-controlled buildups of the Lusitanian basin. Salt movement probably did not influence the Baltimore Canyon buildups; comparable, uniform subsidence histories may explain their similarity to the Portuguese occurrences.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.