Stratigraphy and sedimentology of Pliocene strata in the forearc basin (Waikoau and Waikari River catchments), northern Hawke's Bay
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An area of over 250 km² in northern Hawke's Bay has been geologically mapped at a scale of 1:50 000, and over 1350 m of Pliocene sedimentary section described and interpreted in the Waikoau and Waikari River catchments. Two new formations and 13 members have been defined, and two formations redefined. The strata show little structural deformation and form a shallow to moderately dipping (5°- 25°) succession on the uplifted limb of the Hawke's Bay Monocline. The rocks are of Pliocene age, ranging from Opoitian to Nukumaruan. The formations show rapid changes in thickness and lithology along strike from the southwest to the northeast, which is oblique to the depositional paleoslope. Coarse-grained lithologies such as conglomerates and limestones typically become finergrained, less continuous and thinner towards the northeast. In ascending order the new formations ( except where redefined) are: Titiokura Formation, Rangiora Formation, Matahorua Formation (redefined) and Waipunga Formation (redefined). Titiokura Formation is dominated by calcareous siliciclastic sandstone and sandy limestone. The Rangiora Formation and Waipunga Formations are siliciclastic sandstone and siltstone dominated. Matahorua Formation is dominated by siliciclastic sandstone and greywacke conglomerate. Lithofacies analysis has been applied to the strata to establish the deposition paleoenvironments. Four lithofacies associations (siltstone, sandstone, conglomerate and limestone) and 13 lithofacies have been identified. Interpreted bathymetric ranges for the siltstone association ranges from inner to outer shelf. The sandstone association is characterised by massive to tabular cross-bedded sandstone interpreted to have been deposited on a shoreface to innershelf setting; storm current and tidally emplaced tempestites and tidalities were deposited on the lower-shoreface to mid shelf; massive to horizontally bedded sandstone and silty beds were deposited on the inner-mid shelf. Conglomerate associations are characterised by non-marine greywacke conglomerates deposited on a broad braidplain. Sandstone, siltstone and pumice conglomerate all within a calcareous sandy matrix, are interpreted to have been deposited in tidal channels or on a beachface. The limestone association is characterised thick successions (10 - 50 m) of sandy coquina that probably formed on the inner shelf possibly on a structural high as carbonate source areas. Thinner correlative units (50 cm - 6 m) of trough cross-bedded sandy coquina accumulated in inner-mid shelf environments, having been reworked across the shelf from the carbonate source areas. The successions in the four stratigraphic formations have been interpreted within a sequence stratigraphic framework and 25 sequences have been identified. Interpretation of sequence boundaries is based on observed unconformities between lithologies, inferred unconformities between lithologies of differing paleoenvironments, and correlative conformities. Four typical sequence stratigraphic motifs have been interpreted from these formations. Each motif comprises three or four systems tracts: trangressive (TST), highstand (HST), regressive (RST) and lowstand system tracts (LST). Motif 1 is representative of the Titiokura Formation, and consists of sandstone and mud drapes (TST and HST), with sandstone grading up into limestone (RST) having unconformable tops. Motif 2 is represent by the Rangiora Formation. This formation is comprised of thick successions (100 m +) of interbedded sandstone and siltstone (TST), siltstone (HST), and siltstone coarsening up into sandstone (RST). The boundary on top of the RST sandstone is typically unconformable. Motif 3 occurs in the Matahorua Formation. Siltstone or sandstone (TST), sandstone (HST), and interbedded siltstone and sandstone (RST) coarsen up into greywacke conglomerate (LST). The sequence boundary usually lies within the top of the conglomerate between the coarse massive part and finer bedded conglomerate commonly having a heterogeneous silty-sand matrix; rarely the sequence boundary is at the vert top of the conglomerate unit. Motif 4 is represented by the Waipunga Formation. It is characterized by thin successions (10 m) of interbedded siltstone and sandstone (TST, HST) coarsening upward into sandstone (HST). The stratigraphic evolution of each of these motifs has been interpreted within the context of an across shelf profile. Evolution of the motifs during a sea-level transgression, is generally one of limestone deposition near the highstand shoreline. Thinly bedded tidalities, parasequences and massive sandstone with a weak backlap shellbed occur basinward in the TST. Limestone deposition near the highstand shoreline, and siltstone and thickly bedded tidalities basinward, characterise deposition during a sea-level highstand. Erosion and reworking of sediments basinwards during falling sea level conditions result in the formation of erosional sequence boundaries and the accumulation of a regressive sediment wedge (RST). Sea-level lowstands are characterised by the deposition by greywacke conglomerate on a non-marine braidplain. The Pliocene sedimentary succession on the western limb of the Hawke's Bay Monocline is interpreted as having been deposited in a steadily subsiding seaway (forearc basin) with uplift on its western margin. Strong storm and tidal currents reworked sediment from shallow into deeper water across a shelf. The siliciclastic sediment was sourced from erosion in the hinterland. The compressive component within the plate boundary zone in North Island, increased around 2-3 Ma, resulting in uplift of the Axial Ranges, which became a major sediment source. Sea-level fluctuations throughout the deposition of the sedimentary succession in the study area resulted in the development of cyclothems. The periodicity of the cycles appears to increase from the Opoitian to the Nukumaruan, for which 6th order (41 ka) cyclicity is inferred. The bathymetric range of the cyclothems also appears to increase from the Opoitian to Nukumaruan.
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