Abstract
Dunstan Formation lignite bearing strata within the 1,000m thick Miocene Manuherikia Group of Central Otago represent an important coal resource. Three principal coalfields, Blackstone, Roxburgh and Nevis are identified.
Detailed stratigraphic and sedimentologic data from outcrop and over 120 boreholes were assessed to evaluate Central Otago coalfield stratigraphy and the paleoenvironmental controls governing peat deposition on a broad alluvial plain surrounding a large freshwater basin (Lake Manuherikia).
Six complexly interrelated Dunstan Formation members are identified; each member represents a distinctive depositional system that is subdivided into sequences comprising genetically related lithofacies recognized by an abbreviated mnemonic code. Depositional systems recognized are: 1. Braided river deposits of the St Bathans Member occurring at the base of the group, infilling valleys entrenched in the weathered schist and greywacke basement. They incorporate deposits of major channels and a diverse range of shallow minor channels dominated by bar and dune bedforms. 2. Fine grained alluvial plain deposits of the Fiddlers Member and Middle Old Woman
Submember. The former represents an extensive long-lived fluvial plain traversed by meandering channels dispersing sediment into muddy flood-basins and Lake Manuherikia. Locally, muddy and peat-forming swamps replaced abandoned splay filled flood-basins. 3. Anastomosed fluvial deposits of the Lower Old Woman· Submember. Stabilized river channels were bordered by long-lived muddy flood-basins and peat-forming swamps, producing thick laterally discontinuous deposits of lignite, gravelly sand and mud. 4. Carbonace,ous rich delta plain deposits representing Blackstone and Teviot Members developed at major points of sediment entry into Lake Manuherikia. Complexly interdigitating fine grained facies representing short-lived peat swamps, well drained swamps, interdistributary bays, levee and distributary channels accumulated on the lower delta plain, contrasting with the long-lived peat-forming backswamp environment that dominated the upper delta plain - lower alluvial plain landscape. 5. Marginal laaustrine deposits of the Kawarau Member represent a stagnant swampy shoreline interdigitating with interdistributary bays (Cromwell Submember) and a current agitated terrigenous lake margin (Ewing Submember) of beaches, mudflats, backbarrier lagoons and bays. Ostracods, algae (including oncolites), fish and bird remains are dispersed within shoreline facies. An enlarging Lake Manuherikia progressively onlapped older lake margin and alluvial plain strata, depositing a thick (700m) pile of sandy and muddy (including richlykerogeneous) sediments (Bannockburn Formation), blanketing an area in excess of 5,600 km2.
Extraordinary thick coal beds of the 33 - 90 m thick Johnstone and McPherson Seams (Blackstone and Teviot Members) accumulated in long-lived backswamps of upper delta plain - lower alluvial plain environments. Thin (< 5m thick) laterally discontinuous coal beds with numerous splits occurring in the upper portions of these seams accumulated in short-lived lower delta plain swamps, adjacent to a network of interdistributary bays and distributary channels. Pluvial plain peats primarily accumulated in swamps that enveloped infilled flood-basins of the broad low gradient Fiddlers alluvial plain, or they accumulated (like the deposits of the 24m thick Ben Nevis Seam) in long-lived swamps on the Old Woman alluvial plain, adjacent to stable anastomosed channels. In fluvial settings, coal quality, seam configuration, frequency and thickness of partings was primarily influenced by channel overbank and splay sedimentation. Almost total abandonment of Old Woman flood-plain alluviation and a sustained high water table were conducive to the development of the laterally extensive 22m thick Homestead Seam of the Upper Old Woman Submember. At the lake margin, peat-forming swamps interdigitating with interdistributary bays produced the numerous thin (< 5m thick), laterally discontinuous coal beds constituting the Kawarau Seam. Cyclic repetition of these coal beds was governed by fluctuating lake levels.
The study emphasizes the significance of detailed observations and analyses of coal-bearing strata, demonstrating the potential role of paleoenvironmental modeling in coal exploration and mining activities.