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Title: Matheson Bay report

Research methods report: 

These reports help the writer learn experimental procedures and ways research findings are made in the subject. IMRD (Intro, Methods, Results, Discussion) structure is commonly used but research questions are often provided by the lecturer, and the writers focus on methods, results and discussion. They include Experiment Reports, Field Reports and Lab Reports.

Copyright: Alexandra Serrano

Level: 

Second year

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

Lithologic descriptions:

WAIPAPA GROUP

UNIT A – basement greywacke

  • dark, very fine grained (<1mm), highly fractured, (possibly quartz) veins
  • compaction cleavage, slightly metamorphosed

WAITEMATA GROUP – KAWAU SUBGROUP, CAPE RODNEY FORMATION

UNIT B – lower greywacke breccia

  • poorly sorted, mostly angular clasts, but also rounded
  • clast supported, approximately 20mm coarse pebbles
  • greywacke clasts (Unit A) – monomictic
  • lens-shaped protrusions on northern part

UNIT C – gravelly sandstone, sandy-gravelly conglomerate

  • orthogonal fractures; more lithic, less calcareous
  • largely matrix supported (with up to 30% matrix component in areas with fossils), also clast supported (in areas with coarser sediments)
  • poorly sorted, subrounded clasts à orthoconglomerate
  • present articulated corals, abraded bivalve, gastropod à shallow water
  • thin layer of mudstone

UNIT D – upper greywacke breccia

  • fine-coarse boulder clasts (256mm-1500mm)
  • clast supported, poorly sorted, subangular clasts
  • medium scale (approx. 1m) wedge contact with E further south (lenticular)
  • not exposed or disappeared up north

UNIT E – sandy-gravelly limestone

  • poorly sorted, mostly subrounded clasts, but also subangular coarse pebbles (25-45mm)
  • calcareous algae on seafloor à cement
  • large, thick-shelled bivalve fossil, colonial corals à shallow water
  • lensoid geometry à throughout shore platform

WAITEMATA GROUP – WARKWORTH SUBGROUP, PAKIRI FORMATION

UNIT F – interbedded sandstone and mudstone

  • medium-coarse sand (250-750μm)
  • well sorted, well rounded à texturally mature
  • thickly laminated mst on cliff face (>10mm mudstone - slabby) à turbidite deposit further south
  • thinly laminated on northern part

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 world2, 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.