Evolution of Submarine Channel-Lobe Complex: The Lower Mount Messenger Formation, Taranaki Basin, New Zealand

The nearly continuous outcrops of the Upper Miocene Lower Mount Messenger Formation (LMMF) exposed along the west coast of the North Island, New Zealand, provide an opportunity to study submarine channel-lobe complex at the bed scale. This study provides a link between flow mechanics, processes of sedimentation, and larger-scale architecture of the LMMF.

Newly acquired stratigraphic data, mapping results, and sedimentological analysis suggest that there are 5 lithofacies that comprise the LMMF: (Lf1) mud-clast conglomerate; (Lf2) thick-bedded sandstone (>40 cm); (Lf3) interbedded thin- to medium-bedded (10-40 cm) sandstone and mudstone; (Lf4) mudstone; (Lf5) mass-transport deposits (MTDs). The LMMF is subdivided into 3 facies associations (FA): FA1 comprises channels up to 15 m deep filled with cross-laminated thick-bedded sandstones in a fining- and thinning-upward cycle, FA2 comprises a thinning- and fining-upward cycle with shallow scours and cross-stratified sandstones, FA3 comprises 2-10 m packages of cross-stratified thick-bedded sandstones with mudstone in between every package. FA1 is interpreted as a submarine channels or channel complex, FA2 as a transitional zone between channels and lobes, and FA3 as a lobe complex. Thus, the LMMF consists of 5 lobe complexes (outer fan) separated by MTDs, a transitional zone (middle fan), and 3 submarine channels (inner fan). The depositional style, FAs and flow mechanics of lobes and channels are principally different. The lobes comprise packages of laterally continuous thick cross-stratified sandstones. Channels show a thinning-upward trend with thick cross-laminated sandstones filling the incision. Transitional zone has a similar to channels thinning-upward trend, but has shallow incision and abundant cross-stratification. The FA3 and FA2 were compared quantitatively, these findings suggest that the FA3 lithofacies shows finer grained, thinner sedimentation units deposited by lower energy flows than those represented by FA2 lithofacies. Based on field observations and literature review, we concluded that dunes documented in the LMMF might be ripples but unusually high.

The LMMF represents a progradational cycle with highly channelized and deeply erosive flows in the upper LMMF that formed and bypassed the channels and deposited the sediments in compensational lobes in the lower LMMF. As the sediment input ceased, the poorly erosive flows filled the channels and didn’t reach the site of lobe deposition.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California