Variations in Lowstand Systems Tracts: Constraints on Exploration
BROWN, L. F., JR., Consultant, Georgetown, TX
Results of worldwide exploration of lowstand systems tracts support continued application and evaluation of Exxon's cyclic sequence concepts but indicate the need for a better understanding of erosional and depositional variations possible along ancient lowstand coastlines. Exxon's idealized siliciclastic (type 1) model applies where a major highstand fluvial system was entrenched during falling relative sea level, eroding canyons and contributing sediments to lowstand depositional systems. Canyons and incised valleys were filled by late lowstand and retrogradational (transgressive) systems. Not explicit in Exxon's scenario are lowstand tracts at sites of minor entrenched coastal-plain streams or along interdeltaic or nondeltaic margins. Also, higher-order systems tracts may vary temp rally and spatially in response to respective positions on falling and rising limbs of lower-order cycles.
A spectrum of systems tracts, identified along ancient basin margins, provides clues for predicting lowstand targets. In the absence of rivers, basin-floor sediments were supplied locally by headward-slumping submarine canyons and erosion of contributary valleys into subaerially exposed highstand shelf and/or strandline systems. Submarine erosion typically continued during subsequent rise and highstand of sea level, and sediments may have been introduced to basin floors through canyons from active retrogradational and highstand longshore systems. Headwardly eroded canyons and valleys were not always filled during subsequent transgression and highstand, leading to long-term multiple erosional/depositional cycles to produce some of the world's major ancient canyon complexes.
The type and distribution of highstand systems tracts strongly influenced the quality and distribution of sandstone reservoir potential in subsequent lowstand tracts and, therefore, may help guide deep-water exploration along ancient basin margins.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)