CYCLICITY AND HIERARCHY OF THE DEEP-WATER CHANNEL-LOBE COMPLEX OF THE LOWER MOUNT MESSENGER FORMATION, NEW ZEALAND

The outcrops of the Lower Mount Messenger Formation (LMMF) in Taranaki region, New Zealand, provide an opportunity to study an ancient submarine channel-lobe complex at the bed scale. The deposits of the LMMF show cycles both on a bed scale and on large scale architectural elements.

The LMMF is subdivided into 3 facies associations (FA). FA1 comprises repeating packages of flat thick-bedded sandstone with dunes topped by scours filled with thin-bedded sandstone and draped by mudstone. FA1 represents a lobe complex. FA2 comprises thinning- and fining-upward sequences of mud-clast conglomerate, thick-bedded sandstone with dunes topped by packages of thin-bedded sandstone and mudstone. Mostly shallow scour marks the bases of FA2. FA2 represents proximal splays and are up-slope equivalent of the FA1 deposits. FA3 comprises thinning- and fining-upward sequences with mud-clast conglomerate, thick-bedded sandstone with climbing ripples, and thin-bedded sandstone and mudstone, topped by mudstone. FA3 is contained within large erosional surfaces and represents channel-fill sequences.

Quantitative analysis of FA1 and FA2 revealed: (1) there is an overall thinning-upward thickness trend in the FA2 section, whereas FA1 shows irregular bed thickness trends; (2) the sand is slightly coarser in FA2; (3) the bioturbation intensity is lower in FA2; (4) the average thickness of T_d-edivisions is higher and sandstone beds tend to be thicker in FA2; and (5) a fining-upward grain-size trend within individual sedimentation units is better developed in FA2 than in FA1, where beds show almost no grain-size trends.

Channels are full of climbing-ripple cross-lamination; lobes are full of climbing-dune cross-stratification. This reflects that the lobe was formed by energetic, collapsing but still bypassing downslope flows. The channels were filled by waning flows at the final stage of their evolution. When the lobes were formed, the flows largely bypassed the channels without leaving any sediment except probably lag deposits of mud clasts and deposited the sediments in the lower LMMF. As the sediment input ceased, the flows started to fill the channels and didn’t reach the site of lobe deposition. The channels were filled, the whole systems shifted or shut-off, and the thick mudstone interval deposited on top of the LMMF.