AWA: Academic Writing at Auckland
Title: Matheson Bay report
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Copyright: Alexandra Serrano
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Description: Detailed geology map of the shore platform, sedimentologic logs, lithologic descriptions of each unit, 1-2 page summary of Matheson Bay geology and paleoenvironment.
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Matheson Bay report
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Lithologic descriptions: WAIPAPA GROUP UNIT A – basement greywacke
WAITEMATA GROUP – KAWAU SUBGROUP, CAPE RODNEY FORMATION UNIT B – lower greywacke breccia
UNIT C – gravelly sandstone, sandy-gravelly conglomerate
UNIT D – upper greywacke breccia
UNIT E – sandy-gravelly limestone
WAITEMATA GROUP – WARKWORTH SUBGROUP, PAKIRI FORMATION UNIT F – interbedded sandstone and mudstone
Matheson Bay Geology and Paleoenvironment The unit sequence can be divided into two depositional environments created by active tectonism, shallow continental shelf or deep benthic zone. Abrupt vertical and lateral changes in the lithofacies indicate this transition in depth. Basement greywacke formed by accumulation and compression of fine-grained sediments, indicating a low energy benthic zone or abyssal plain. It had undergone intense deformation that forced fluid to move through the planes of weakness in the rocks (compaction cleavage), thereby creating quartz veins (Ballance, 1993). It can only be found in the northern area of Matheson Bay as protrusions and underlying Unit C. The Kawau subgroup demonstrates the transgression and subsidence in the Waitemata Basin during the Miocene period. It directly overlies the irregular basement greywacke. It consists mainly of local, paralic conglomerate/breccia and is on shallow photic zone on the continental shelf. During early Miocene, the movement between the Pacific and Indian plates caused lateral displacement (or thrusting) that formed the Northland Allochthon (Hayward, 1993). This formation covered most of eastern continental shelf. The basement terrane that had been uplifted subsequently eroded aerially as evidenced by the angular greywacke breccia (Unit B). It composes of lithified poorly sorted, coarse pebbles to boulders, which are attributes of base-of-cliffs talus deposits. The coastline was characterised by steep cliffs and sea stacks and unit B as infill to topographic lows. Conglomerate (Unit C) has the highest fossil assemblage in all units. It exhibits poorly sorted, subrounded clasts, caused by transportation of sediments through storm-generated waves. In which, these waves could also result in the lateral grading of grains (Ricketts, & et al, 1989). Coarse-grained conglomerate can be found in the north, whereas coarse sandstone in the south. Transgression enabled epifaunal species to thrive under shallow, sunny and high energy environments. Thus, fossils of colonial corals, brachiopods, echinoderm spines, gastropods and bivalve all indicate this transition from deep to shallow water paleoenvironments. Barnacles, gastropods, echinoderms and brachiopods are filter feeders that require high wave action and tropical warm waters. These can be found within the northern part of the beach. Also, rhodolith corals were attached to substrate of less than 20m in the photic zone. In contrast, some colonial corals have narrow stems that grow upwards and outwards in low light environment, such as shady caves. This could mean continued subsidence and submarine erosion. Greywacke breccia (Unit D) has little terrigenous input, wherein the sediments originated from obduction or thrusting of allochthon that renewed the uplift (Hayward, 1993). Therefore, the subduction east of New Zealand led to series of earthquakes, which boulder breccias are eroded and derived from. Coarse, bioclastic limestone (Unit E) accumulated at intertidal to upper bathyal depth, wherein thick-shelled bivalves proliferating in high energy environments were found. Further south, approximately 50mm of siltstone overlie the limestone. It is characteristically green in colour because of the mineral glauconite. It indicates shallow continental shelf marine deposition, with slow deposition, tranquil waters and depth less than 400m. However, the barnacle Bathylasma aucklandica dwelled in deep water, low energy environment and is found in association with it. The fossils must have dislodged after erosion. The thin layer of siltstone means it deposited in an abrupt short period of time. Hence, there could be a submarine slump, with bivalves only included within the sediment load, after sudden subsidence. These shallow marine sediments are overlain by interbedded sandstone and mudstone Waitemata group (Unit F). Continued subsidence means sediment starvation within the basin, intervals of turbidity currents deposited layers of sandstone/mudstone as infill (Hayward, 1993). Afterwards, suspended sediment and current action also deposited Unit F for an extended period of time in between turbidity current (Balance, 1993). Coarse-grained, sandstone predominantly occurs as a thin layer across the extent of Matheson Bay, with only the southern end of the beach the thick, weakly laminated mudstone can be found. This means there is a slope in the south direction. References Ballance, P. F. (1993). Geological history and structure of the Auckland district. A natural history of Auckland. David Bateman Ltd in association with Auckland Regional Council, 4-11. Hayward, B. W. (1993). The tempestuous 10 million year life of a double arc and intra-arc basin - new zealand’s northland basin in the early miocene. Sedimentary basins of the world, 2, 113-142. Ricketts, B., Ballance, P., Hayward, B., & Mayer, W. (1989). Basal waitemata group lithofacies: rapid subsidence in an early miocene interarc basin, New Zealand. Sedimentology, 36(4), 559-580. |
