MICROSTRAT
Megaspores - what are they?
Megaspores are large plant spores (up to 1mm in diameter), made of a tough biopolymer which are easily extracted using standard micropal. processing methods. They were produced by small, herbaceous plants (lycopsids) which housed two types of sporangia producing microspores and megaspores. Dispersal and germination of these spores required a fresh-water medium and so they are fresh-water indicators in the sedimentary record, typically associated with fluvio-deltaic facies.
The modern megaspore-producing plant analogue is the simple moss, Selaginella – see below.
Profile across a typical Jurassic delta-plain showing habitats supporting lycopsid floras
Megaspores - Stratigraphic application
Since 2008 MicroStrat in collaboration with Statoil /Equinor AS has analysed over 120 exploration and production wells from across the Norwegian shelf, including the Barents Sea. This together with outcrop studies (e.g. Svalbard, Yorkshire,UK) has led to the development of a high resolution regional Triassic – Jurassic megaspore biozonation scheme which is not only a proxy for palynofloral-based schemes, but in many cases can provide superior resolution (e.g. Båt, Statfjord groups). The biostratigraphic evaluation of fluvio-deltaic sequences should ideally involve both megaspore and palynofloral analysis for cross-calibration, and to optimise resolution.
Megaspores (x40) from the Rhaetian Banksisporites pinguis Zone, Statfjord Group, NOCS
Palaeoenvironmental indicators
Lycopsid plants that produced megaspores required specific conditions to colonise and reproduce: this ecological ‘envelope’ essentially followed delta-top environments in time and space, and specifically the lake-mire habitat where megaspores were generated and dispersed. The frequency of megaspores, small-spores and associated phytoclasts can be used to characterise delta-plain environments, and in conjunction with marine microfaunas define marine-brackish-freshwater trends associated with progradational depositional cycles (e.g. Brent Group - see below).
Wire-line/lithology profile through a deltaic progradational section showing microfaunal - megaspore distribution, Brent Group, well 34/10-B-12, Gullfaks Field
Biostratigraphic well correlation - Åre Formation, Haltenbanken, NOCS