Recognition and distinction of normal and forced regression in cyclothemic strata: a Plio-Pleistocene case study from eastern North Island, New Zealand
Douglas W. Haywick, 2000. "Recognition and distinction of normal and forced regression in cyclothemic strata: a Plio-Pleistocene case study from eastern North Island, New Zealand", Sedimentary Responses to Forced Regressions, D. Hunt, R. L. Gawthorpe
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The Tangoio block of eastern North Island, New Zealand contains an exceptionally well exposed, 550 m thick sequence of Plio-Pleistocene cyclothemic sedimentary rocks, the Petane Group, that were deposited along the western margin of a shallow (<200 m) pericontinental seaway. The cyclicity that characterizes the Petane Group in this area was generated through recurring 40 ka (sixth-order) sea-level changes of c. 75–150 m. Data from more than 100 sections indicates that the strata consist of five distinct cyclothems, each consisting of a lower formation of mudstone deposited in midshelf environments (highstand system tract, HST), and an upper formation dominated by coarse-grained facies deposited in shallow marine and occasionally non-marine environments. Western exposures of HST mudstones in the lower portion of the Petane Group shoal up into shallow marine siliciclastic sandstones, fluvial gravels and siltstones. The transition is gradational and is consistent with progradation of a siliciclastic shoreline during late sea-level highstand. The shoaling upward siliciclastic sandstone interval represents a regressive system tract (RST) whereas the fluvial beds represent a lowstand system tract (LST). LST gravel beds are sharply overlain by a transgressive surface of erosion and fining-upward siliciclastic sandstone facies of a transgressive systems tract (TST).
Carbonate sediments replaced siliciclastic sandstones as the dominant coarse-grained lithology in the upper Petane Group, probably due to bypass of siliciclastic sediment during sea-level fall. Transitions from HST mudstone into carbonate sand and bioclastic limestone (coquina) are generally sharp everywhere except the downdip eastern portions of the Tangoio block. The contacts represent regressive surfaces of erosion produced during falling stages of sea-level and/or ravinement surfaces formed during rising phases of sealevel (transgressive surfaces of erosion). Carbonate facies atop these erosional surfaces are generally interpreted as TST deposits, except for the uppermost limestones in the Petane Group that occur below a sequence boundary and therefore represent a forced regression systems tract (FRST). Stratal position is the only distinguishing characteristic of FRST limestones in the Petane Group, otherwise they are identical to TST deposits. In eastern portions of the Tangoio block, HST mudstones pass gradationally into RST calcareous sandstones and LST limestones. Systems tracts here are bounded by correlative conformities rather than erosive surfaces indicating continuous sedimentation during sea-level fall and lowstand. Differences in sedimentation and subsidence rates between carbonate and siliciclastic sedimentary systems, are responsible for the different stratal architecture of systems tracts in the Petane Group.
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An increasing number of studies in recent years have demonstrated that significant progradation of shallow marine systems occurs under conditions of base-level fall. These new data are forcing many sedimentary geologists to critically re-evaluate many aspects of sequence stratigraphy relating to erosion and deposition during base-level (lake- or relative sea-level) fall, and the intrinsic link made between stratal geometries and base-level change. For the first time, this volume brings together a collection of articles that focus solely on forced regressions, providing a more complete picture of the development, formation, variability and preservation of the surfaces and deposits generated during base-level fall.
The results of the studies published here will be of interest to all geologists attempting to understand the relationship between changes in base-level and stratigraphy, and to all who use sequence stratigraphy as a method of stratigraphic correlation and interpretation at outcrop and in the subsurface.