Submarine Canyons as High Frequency Sequence Boundaries in the Niger Delta, Nigeria

McHargue, Tim R.¹ (1) Chevron Energy Technology Company, San Ramon, CA

In Miocene neritic sediments of the Niger Delta, numerous submarine canyon systems are interpreted as sequence boundaries in both low- and high-frequency sequences.

This paper focuses on a single low frequency sequence, which ranges in thickness from about 300m to over 800m in the area of study. The basal sequence boundary is a submarine canyon system several kilometers wide and hundreds of meters deep. Muddy canyon fill represents the lowstand systems tract (LST). The rest of the sequence consists of a sand-rich aggradational to retrogradational sequence set that is interpreted as a transgressive systems tract (TST). Maximum faunal abundance and diversity occur near the top of the sequence.

Each high-frequency sequence begins with the development of a small submarine canyon. Away from the canyon, each high-frequency sequence may be as much as 90 m thick. Each high-frequency sequence consists of muddy canyon fill (LST) overlain by an upward-decreasing abundance of shelf sand (TST) followed by a widespread marine shale (MFS), and finally by an interval with an upward-increasing abundance of shelf sand (HST).

The observed pattern of nested sequence frequencies is very orderly and provides a highly predictive model of sediment stacking pattern. For each younger high-frequency sequence: 1) total thickness decreases; 2) erosional relief at the basal sequence boundary decreases; 3) the highstand systems tract makes up a smaller proportion of the total thickness of each sequence; and 4) the total faunal abundance associated with each MFS increases upward to a maximum at the low-frequency MFS.