Abstract: Sequence Stratigraphy in a Mixes Garbonate-Siliciclastic Setting: The Upper Jurassic of Portugal

Chris Wilson, R.R. Leifelder

The Upper Oxfordian to Tithonian fill of the southern part of the Mesozoic Lusitanian basin contains several mixed carbonate-siliciclastic assemblages that accumulated during extremely rapid rates of rift-related basement subsidence and subsequent slow regional subsidence. Prior to this study, it appeared that the depositional sequences ZA 4A, 4.5 and 4.6 shown on the original Exxon global chart for the late Oxfordian-Kimmeridgian could not all be identified in the basin, yet there is good evidence for the presence of the series of sequence boundaries shown in the chart for the Tithonian. Could the very high tectonic subsidence rates over the older interval (approx. 250 m/10⁶ yr, and as high as 800 m/10⁶ yr in the depocentre of the Arruda sub-basin) have masked a y eustatic effects, or had our earlier work integrating seismic and sedimentologic data failed to recognize additional sequence boundaries?

More detailed work has shown that it is likely that all the upper Oxfordian to upper Tithonlan third order sequence boundaries identified on the revised cyde chart for the Jurassic are present in the Arruda sub-basin. However, not all of the sequences can be resolved in the three main tectonic settings of half-graben basin centre, and basin bounding footwall block and salt pillow. A second order sequence is dearly identifiable on seismic sections, and as a transgressive--regressive trend at outcrop; it is related to the late Jurassic rifting event that heralded ocean opening in the Tagus abyssal plain to the west.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994