title: Geology of the western Mamaku Plateau and variations in the Mamaku ignimbrite

authors: Fransen, Pieter J.B.

subjects: Geology

abstract: Pleistocene ignimbrites in the western Mamaku Plateau east of Putaruru consist of the following successively younger formations: (?) Ongatiti Ignimbrite, Ahuroa Ignimbrite, Whakamaru Ignimbrite, Waihou Ignimbrite (new), Waimakariri Ignimbrite (new), Mamaku Ignimbrite. They can be distinguished on the basis of their compositional-, welding-, textural-, and field characteristics. The ignimbrites are separated by unconformities, and intercalated fluvial sedimentary deposits. The three most important units are the Whakamaru, Waimakariri and Mamaku Ignimbrites, which are well exposed and widespread.

The Mamaku Ignimbrite covers a surface area of nearly 4300 km² on the Mamaku Plateau. Corresponding vertical changes in lithology, petrography and physical properties near the Rotorua Caldera define two flow members: a lower Sheet 1, and an upper Sheet 2, each of which is about 60 m thick. The ignimbrite constitutes a simple cooling unit which indicates that Sheet 2 was emplaced shortly after Sheet 1. Only one sheet (? Sheet 1) is evident in the western Mamaku Plateau. Welding zonation near-to-source is characterised by a gradual change from a basal glassy lenticulite zone to a nonwelded top. Smith's (1960b) zonal classification ranks the ignimbrite as being partially welded. This condition suggests that total thickness is not necessarily a major factor in the degree of welding of ignimbrites. 

The phenocryst assemblage of plagioclase, quartz, pyroxene, opaques, hornblende and biotite, is set in a devitrified fine ash matrix, which is an ubiquitous feature of the ignimbrite. Higher modal phenocryst content at the base of each sheet is attributed to compaction. Overall the phenocrysts are uniformly distributed and there is little vertical variation. Seven bulk chemical analyses reveal only minor vertical variations in composition. The near uniform chemical and mineralogical trends suggest nondifferentiation of the source magma. 

Texturally the Mamaku Ignimbrite is a poorly sorted lapilli ash. The grading of fragments like that shown in "the standard ignimbrite flow unit" is absent. However, the presence of a zone of aligned pumice fragments indicates that laminar flow operated during the later stages of deposition. Two major inferred faults in the western Mamaku Plateau define a western horst, a central graben, and an eastern horst. The eastern horst is postulated to be the southern extension of the axial median horst in the Hauraki Depression and of the Hauraki Rift. 

Repeated and extended intervals of erosion followed the emplacement of the ignimbrites. Detritus from the eroding sheets was removed by rivers flowing into the Hauraki Depression. Construction of the Mamaku Plateau has largely taken place since the eruption of the Whakamaru Ignimbrite, c.300,000 years ago, and terminated with the eruption of the Mamaku Ignimbrite, c.l40,000 years ago. Excavation of the long deep valleys in the plateau occurred mainly in the latter 42,000 years.

text: Geology of the western Mamaku Plateau and variations in the Mamaku ignimbrite Pleistocene ignimbrites in the western Mamaku Plateau east of Putaruru consist of the following successively younger formations: (?) Ongatiti Ignimbrite, Ahuroa Ignimbrite, Whakamaru Ignimbrite, Waihou Ignimbrite (new), Waimakariri Ignimbrite (new), Mamaku Ignimbrite. They can be distinguished on the basis of their compositional-, welding-, textural-, and field characteristics. The ignimbrites are separated by unconformities, and intercalated fluvial sedimentary deposits. The three most important units are the Whakamaru, Waimakariri and Mamaku Ignimbrites, which are well exposed and widespread.

The Mamaku Ignimbrite covers a surface area of nearly 4300 km² on the Mamaku Plateau. Corresponding vertical changes in lithology, petrography and physical properties near the Rotorua Caldera define two flow members: a lower Sheet 1, and an upper Sheet 2, each of which is about 60 m thick. The ignimbrite constitutes a simple cooling unit which indicates that Sheet 2 was emplaced shortly after Sheet 1. Only one sheet (? Sheet 1) is evident in the western Mamaku Plateau. Welding zonation near-to-source is characterised by a gradual change from a basal glassy lenticulite zone to a nonwelded top. Smith's (1960b) zonal classification ranks the ignimbrite as being partially welded. This condition suggests that total thickness is not necessarily a major factor in the degree of welding of ignimbrites. 

The phenocryst assemblage of plagioclase, quartz, pyroxene, opaques, hornblende and biotite, is set in a devitrified fine ash matrix, which is an ubiquitous feature of the ignimbrite. Higher modal phenocryst content at the base of each sheet is attributed to compaction. Overall the phenocrysts are uniformly distributed and there is little vertical variation. Seven bulk chemical analyses reveal only minor vertical variations in composition. The near uniform chemical and mineralogical trends suggest nondifferentiation of the source magma. 

Texturally the Mamaku Ignimbrite is a poorly sorted lapilli ash. The grading of fragments like that shown in "the standard ignimbrite flow unit" is absent. However, the presence of a zone of aligned pumice fragments indicates that laminar flow operated during the later stages of deposition. Two major inferred faults in the western Mamaku Plateau define a western horst, a central graben, and an eastern horst. The eastern horst is postulated to be the southern extension of the axial median horst in the Hauraki Depression and of the Hauraki Rift. 

Repeated and extended intervals of erosion followed the emplacement of the ignimbrites. Detritus from the eroding sheets was removed by rivers flowing into the Hauraki Depression. Construction of the Mamaku Plateau has largely taken place since the eruption of the Whakamaru Ignimbrite, c.300,000 years ago, and terminated with the eruption of the Mamaku Ignimbrite, c.l40,000 years ago. Excavation of the long deep valleys in the plateau occurred mainly in the latter 42,000 years.

year: 1982

title: Generating accurate rule sets without global optimization

authors: Frank, Eibe

subjects: computer science

abstract: The two dominant schemes for rule-learning, C4.5 and RIPPER, both operate in two stages. First they induce an initial rule set and then they refine it using a rather complex optimization stage that discards (C4.5) or adjusts (RIPPER) individual rules to make them work better together. In contrast, this paper shows how good rule sets can be learned one rule at a time, without any need for global optimization. We present an algorithm for inferring rules by repeatedly generating partial decision trees, thus combining the two major paradigms for rule generation-creating rules from decision trees and the separate-and-conquer rule-learning technique. The algorithm is straightforward and elegant: despite this, experiments on standard datasets show that it produces rule sets that are as accurate as and of similar size to those generated by C4.5, and more accurate than RIPPER’s. Moreover, it operates efficiently, and because it avoids postprocessing, does not suffer the extremely slow performance on pathological example sets for which the C4.5 method has been criticized.

text: Generating accurate rule sets without global optimization The two dominant schemes for rule-learning, C4.5 and RIPPER, both operate in two stages. First they induce an initial rule set and then they refine it using a rather complex optimization stage that discards (C4.5) or adjusts (RIPPER) individual rules to make them work better together. In contrast, this paper shows how good rule sets can be learned one rule at a time, without any need for global optimization. We present an algorithm for inferring rules by repeatedly generating partial decision trees, thus combining the two major paradigms for rule generation-creating rules from decision trees and the separate-and-conquer rule-learning technique. The algorithm is straightforward and elegant: despite this, experiments on standard datasets show that it produces rule sets that are as accurate as and of similar size to those generated by C4.5, and more accurate than RIPPER’s. Moreover, it operates efficiently, and because it avoids postprocessing, does not suffer the extremely slow performance on pathological example sets for which the C4.5 method has been criticized.

year: 1998-01

title: Indigidata Aotearoa post-event summary report 2023

authors: Te Kotahi Research Institute

subjects: Indigenous Data Sovereignty

abstract: From the 21st to the 24th of August, the inaugural Indigidata Aotearoa 2023 was held at Te Whare Wānanga o Waikato. In recent years there has been significant advances in the fields of Indigenous data science and sovereignty. The Indigidata Aotearoa programme was designed to develop an understanding of Indigenous data science and sovereignty alongside some of the best researchers in Aotearoa New Zealand. The fully funded wānanga was open to Māori (tauira, kaitiaki, kaipakihi) participants from across Aotearoa. Applications were received from more than 35 respondents, and 23 tauira Māori attended the inaugural event.

text: Indigidata Aotearoa post-event summary report 2023 From the 21st to the 24th of August, the inaugural Indigidata Aotearoa 2023 was held at Te Whare Wānanga o Waikato. In recent years there has been significant advances in the fields of Indigenous data science and sovereignty. The Indigidata Aotearoa programme was designed to develop an understanding of Indigenous data science and sovereignty alongside some of the best researchers in Aotearoa New Zealand. The fully funded wānanga was open to Māori (tauira, kaitiaki, kaipakihi) participants from across Aotearoa. Applications were received from more than 35 respondents, and 23 tauira Māori attended the inaugural event.

year: 2023