Heterogeneity and Grain Size Distribution of Slope Channel Fill, Miocene Urenui Formation, Taranaki Basin, New Zealand

K. L. Maier¹, P. R. King², and S. A. Graham^1
1Stanford University, Stanford, CA, [email protected], [email protected]
²GNS Science, Lower Hutt, NZ, [email protected]

The Urenui Formation is a Miocene age, mid-slope, deep-water succession located in the Taranaki Basin, New Zealand. This study integrates data at multiple scales to interpret depositional architectures, sedimentation patterns, and factors influencing deposition within the Urenui Formation. Seismic-reflection data of the Urenui Formation are interpreted as recording prograding clinoforms above a large mass transport complex. The Urenui Formation is exposed along the modern-day coastline and correlates with the mid to upper slope sections of the clinoforms based on calibration with well, seismic and paleontological data. The coastal outcrop contains discrete channel complexes at different stratigraphic levels. Each channel has an erosive base and heterogeneous fill deposits, including some portion of thin-bedded fill with minimal erosion, as evidenced by frequent preservation of the tops of ripple beds and starved ripples. Grain sizes in the channel fill deposits are restricted to silt, very fine to medium sand, granule extra-basinal conglomerate, and larger intra-basinal conglomerate. However, grain sizes other than silt are rarely found outside channel confinement. The siltstone surrounding the channel deposits is heavily bioturbated and either massive or slumped.

The exposed erosional channels within the Urenui Formation are inferred to have been back-filled by splay deposits. The thin-bedded channel-fill represents the toe of turbidity current splays with little associated erosion. Once filled and healed, the channels were encased by large amounts of background fine-grained sediment forming the prograding clinoforms. Though the paleobathymetry of the outcrop succession generally shallows up-section, there are no clear trends in grain size or sedimentary structures between the outcrop channels, indicating a heterogeneous slope system. Slumped and deformed sediments are common in this and other slope systems and may have contributed to the formation of erosional channel bases in the Urenui Formation. Levee morphologies are present in seismic-reflection data but have not been identified in outcrop.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009