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The megaspore big show: application

Even before my meeting with Professor David Batten (see previous blog entry The megaspore big show: discovery), the word ‘megaspore’ had reached the ears of the Gullfaks Field team within Statoil, who were about to start a major re-evaluation of the Statfjord Formation reservoir model. The hypothesis that the megaspore biozonation defined in mid-Norway had regional application was about to be tested through a pilot study 580 km further south.

It was late on a Friday afternoon when I identified the first megaspores from a key Gullfaks well. After that, the growing excitement had to be reined in before I made the phone call to Statoil. What followed, through the spring and summer of 2007, I now recall as a blur of sample crates arriving, rounds of sample processing, picking, and analysis, all to meet impossible deadlines. During the subsequent three phases of work, with assistance from Jake Jacovides, a total of 1600 samples were analysed from 55 wells. An effective biozonal framework was established, enabling the Gullfaks team to make major advances in reservoir modelling. It was a landmark event — megaspores had come of age in the world of applied biostratigraphy.

Other fields producing from the Statfjord Formation have since come under scrutiny, including Snorre, Oseberg, and more recently Johan Sverdrup. In all these fields the original megaspore biozonation scheme has been applied and refined, with new marker species identified. The scope of megaspore research was also extended northwards into the Barents Sea. Here pilot studies not only proved up the regional biozonation, but showed that in this basin, lycopsid megaspore-producing plants thrived at an earlier time in the Late Triassic. Lycopsids are simple, low-lying plants which house two types of sporangia producing either megaspores or microspores. The evidence was building that lycopsid floras continued to evolve through the Triassic and Jurassic, dispersing between basins and regions. But how and why did lycopsids thrive so long beyond their alleged zenith in the Carboniferous, and what were the common factors controlling their distribution and dispersal?

Regional work on the Åre Formation demonstrated that in successive biozones, megaspore acme events are associated with coal and rootlet beds, characteristic of delta-top, peat-mire facies. It was also evident that the Rhaetian–Hettangian shift in climate was critical, with the development of wet and humid conditions favouring lycopsid floras. But humidity was not the only factor determining megaspore generation because the host plants required a colonizing platform — the delta top and associated environs. In the Jurassic of the Eurasian region, deltaic development mostly ties into sea level low-stands, the Early Hettangian being a case in point. The study of megaspores in the Barents Sea showed that these conditions prevailed at an earlier time in the Late Triassic. Here a distinctly different megaspore microflora had evolved, with new zonal species.

Morris_chart.png

Regional data demonstrated that lycopsid floral distribution closely followed delta-top facies through space and time; in effect, this plant group colonized and evolved within an envelope that met the optimum conditions. Megaspore count data showed that once this envelope was established megaspore production was massive: over 1500 per 100 g of sediment. From the delta top, megaspores were then dispersed throughout the upper and lower delta-plain channel systems, along with millions of their microspore counterparts (often lumped with Deltoidospora miospores). The ‘megaspore big show’ was repeated again in the Middle Jurassic, when due to thermal uplift localized fluvial-deltaic systems prograded simultaneously south into the Cleveland Basin and north into the Viking Graben. Yet again a changeover of lycopsid floras is evident from the new megaspore species appearing. This pattern appears to have continued into the Early Cretaceous of the Weald Basin: after that, along with other ancient plant groups, the lycopsids went into decline — the era of the angiosperms had begun.


Further reading

Morris, P H, A Cullum, M Pearce, and D Batten (2009). Megaspore assemblages from the Åre Formation (Rhaetian–Pliens- bachian) off shore, mid-Norway, and their value as field and regional stratigraphic markers. Journal of Micropalaeontology, 28 (2), 161–181, DOI 10.1144/jm.28.2.161.

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