Tag: Sedimentation

Conditions that may have underpinned the ‘Cambrian Explosion’

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Geologists of my generation leaned that the earliest signs of abundant and diverse animal life were displayed by an extraordinary assemblage of fossils in a mudstone exposure high on a ridge in the Rocky Mountains of British Columbia. The Burgess Shale lagerstätte, or ‘site of exceptional preservation’, was discovered by Charles Walcott in 1909. It contained exquisite remains, some showing signs of soft tissue, of a great range of animals, many having never before been seen. Though dated at 509 Ma (Middle Cambrian) it was regarded for much of the 20th century as the sign of a sudden burgeoning from which all subsequent life had evolved: the Cambrian Explosion. Walcott only scratched the surface of its riches, its true wonders only being excavated and analysed later by Harry Whittington and his protégé Simon Conway Morris of Cambridge University. Their results were summarised and promoted in one of the great books on palaeontology and evolutionary biology, Wonderful Life (1989) by Steven Jay Gould.

Harbingers of animal profusion first appear around 635 Ma in the Late Neoproterozoic as the Ediacaran Fauna, with the oldest precursors turning up around a billion years ago in the Torridonian Sandstone Formation of northern Scotland. The evolutionary links between them and the Cambrian Explosion are yet to be documented, as creatures of the Ediacaran remain elusive in the earliest Phanerozoic rocks. As regards the conditions that promoted the explosion of animal faunas, the Burgess Shale is a blank canvas, for its riches were not preserved in situ, but had drifted onto deep, stagnant ocean floor to be preserved in oxygen-poor muds that enabled their intricate preservation. The animals could not have lived and evolved without abundant oxygen: what that environment was is not recorded by Walcott’s famous stratigraphic site.

Artistic impression of the Chengjian Biota

China, it has emerged, offers a major clue from around 40 lagerstätten in Chengjian County, Yunnan. They are not only older (518 Ma) than the Burgess Shale but contain 27 percent more faunal diversity: 17 phylums and more than 250 species. Since the discovery of the Chengjian Biota in the first decade of the 21st century palaeontologists have, understandably, been preoccupied by describing its riches in hundreds of scientific papers. The nature of the ecosystem has remained as obscure as that of the Burgess Shale, largely due to the exposed host rocks (laminated siltstones and mudstones) having been weathered. They are superficially similar to the Burgess Shale. In March 2022, 10 scientists working at laboratories in China, Canada, Switzerland and the UK published the results of their painstaking sedimentological investigation of a core dilled through through the entire fossiliferous sequence (Salih, F. and 9 others 2022. The Chengjiang Biota inhabited a deltaic environment. Nature Communications, v. 13, article 1569; DOI: 10.1038/s41467-022-29246-z).

Reconstruction of the near-shore deltaic environment in which the Chengjian Biota lived and evolved. Several rock types and the sedimentary processes that probably formed them shown in ‘cores’ (Credit: Salih et al. Figure 3)

The unweathered core displays a variety of tiny sedimentary structures. These include cross laminations formed by migrating ripples, occasional fine sandstones that include signs of burrowing, graded bedding formed by minor turbidity currents, hummocks formed by back and forth water flow, ripples formed by flow in a single direction and small channels. Unlike the Burgess Shale, the fine-grained Chengjian sediments seem to have been deposited in environments that were far from stagnant and deep. They most closely resemble the offshore parts of the delta of a predominantly muddy river, subject to occasional floods and storms and characterised by large and rapid accumulation of mud and silt by dense sediment-loaded river water flowing down a gently sloping seabed into clearer seawater. That the sediment supply was full of nutrients and oxygen is reflected by small organisms living in burrows. The high-quality preservation of fossils in some layers can be attributed to sudden influxes of freshwater into their marine habitat during storms, so that they were killed in place. Such a near-shore environment, full of nutrients and oxygen but subjected to repeated geochemical and physical stresses, can explain adaptive radiation and evolution at a fast pace. Clearly, that is by no means a full explanation of the Cambrian Explosion, but offers sufficient insight for research to proceed fruitfully.

See also: Modern Animal Life Could Have Origins in a Shallow, Nutrient-Rich Delta, SciTechDaily, 23 March 2022.

The effect of surface processes on tectonics

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Active sedimentation in the Indus and Upper Ganges plains (green vegetated) derived from rapid erosion of the Himalaya (credit: Google Earth)

The Proterozoic Eon of the Precambrian is subdivided into the Palaeo-, Meso- and Neoproterozoic Eras that are, respectively, 900, 600 and 450 Ma long. The degree to which geoscientists are sufficiently interested in rocks within such time spans is roughly proportional to the number of publications whose title includes their name. Searching the ISI Web of Knowledge using this parameter yields 2000, 840 and 2700 hits in the last two complete decades, that is 2.2, 1.4 and 6.0 hits per million years, respectively. Clearly there is less interest in the early part of the Proterozoic. Perhaps that is due to there being smaller areas over which they are exposed, or maybe simply because what those rocks show is inherently less interesting than those of the Neoproterozoic. The Neoproterozoic is stuffed with fascinating topics: the appearance of large-bodied life forms; three Snowball Earth episodes; and a great deal of tectonic activity, including the Pan-African orogeny. The time that precedes it isn’t so gripping: it is widely known as the ‘boring billion’ – coined by the late Martin Brazier – from about 1.75 to 0.75 Ga. The Palaeoproterozoic draws attention by encompassing the ‘Great Oxygenation Event’ around 2.4 Ga, the massive deposition of banded iron formations up to 1.8 Ga, its own Snowball Earth, emergence of the eukaryotes and several orogenies. The Mesoproterozoic witnesses one orogeny, the formation of a supercontinent (Rodinia) and even has its own petroleum potential (93 billion barrels in place in Australia’s Beetaloo Basin. So it does have its high points, but not a lot. Although data are more scanty than for the Phanerozoic Eon, during the Mesoproterozoic the Earth’s magnetic field was much steadier than in later times. That suggests that motions in the core were in a ‘steady state’, and possibly in the mantle as well. The latter is borne out by the lower pace of tectonics in the Mesoproterozoic. Continue reading “The effect of surface processes on tectonics” →

Some cunning radiometric dating

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At the end of the 1970’s I was invited by the Deputy Director of the Geological Survey of India (Southern Region) to participate in the Great Postal Symposium on the Cuddapah Basin: a sort of harbinger of the Internet and Skype, but using snail-mail. Feeling pretty honoured and most intrigued I accepted; not that I knew the first thing about the subject. A regular stream of foolscap mimeographed contributions kept me nipping out of my office to check my pigeon hole for about 6 months. I learned a lot, but felt unable to comment. Four years on I was taken across the Cuddapahs by my first research student – a budding moto-cross driver with a morbid fear of bullock carts – en route from the Archaean low-grade greenstone-granite terrains of Karnataka for a peek at the fabled charnockites near Chennai (then Madras). A bit of a round-about route but spurred by my memories of the Great Postal Symposium. Sadly, the detour was marred for me by a severe case of sciatica brought on by manic driving, the state of the trans-Cuddapah highway and a misplaced gamma-globulin shot to ward off several varieties of hepatitis: I mainly blamed the nurse who demanded that I drop my drawers and bravely take the huge needle in a buttock – they do these things more humanely these days. Anyhow, apart from seeing many dusty villages build of slates perfect enough to make a full-size snooker table, my mind was elsewhere and I have long regretted that.

Landsat image mosaic showing part of the Cuddapah Basin.
Landsat image mosaic showing part of the Cuddapah Basin.

Hosting possibly the world’s only diamondiferous Precambrian conglomerate, the Cuddapah Basin contains a 5 km thickness of diverse sedimentary strata, but no tangible fossils. It rests unconformably on the Archaean greenstone-granite terrain of the Dharwar Craton and so is Proterozoic in age; an Eon that spans 2 billion years. The middle of the lowest sedimentary formations (the Papaghni and Chitravati Groups) contains volcanic rocks dated at ~1.9 Ga; another group is cut by a ~1.5 Ga granite, and hitherto the youngest dateable event is the emplacement of 1.1 Ga kimberlites that sourced the diamonds in the conglomerate. Until recently the stratigraphy has been known in some detail, but how to partition it in Proterozoic time is barely conceivable with just three dates in the middle parts that span 800 Ma. All that can be said about the base of the Cuddapah sediments is that they are younger than the 3.1 to 2.6 Ga Archaean rocks beneath. Since the uppermost beds are truncated by a huge thrust system that shoved deep crustal granulites over them their minimum age is equally vague.

Structurally, the Basin began to form on a stable continent underpinned by the Dharwar Craton, but when that collided with Enderbyland in Antarctica, as part of the accretion of the Gondwana supercontinent, sedimentation may have been in an entirely different setting. Indeed, some of the sediments have been carried over the undisturbed part of the basin by a major thrust system. To explore both sedimentary and tectonic evolution Australian, Indian and Canadian geoscientists combined to sample and radiometrically date the entire pile (Collins, A.S. and 13 others 2015. Detrital mineral age, radiogenic isotopic stratigraphy and tectonic significance of the Cuddapah Basin, India. Gondwana Research, v. 28, p. 1294-1309). By precisely dating detrital micas and zircons from the sediments the team was able to check the source region of sedimentary grains as well as to establish a maximum age for each major stratigraphic unit. This helped establish a 3-part sedimentary and tectonic history. The earliest sediments came from the cratonic area to the west, but there are signs that collisional orogeny between 1590 and 1659 Ma produced a new sedimentary source in metamorphic rocks forming to the east. A return to westward provenance marked the youngest sedimentary setting. This enabled the team to suggest a dual evolution of the Basin, first as an extensional rift opening at the east of what is now the Dharwar craton followed by collisional orogeny that transformed the setting to that of a foreland basin, analogous to the Molasse basin in front of the Alps during Cenozoic times, ending with tectonic inversion when extension changed to compression and thrusting.

But to what extent did the work improve the age subdivision of the Cuddapah Basin? Apparently very little, which may be down to a problem with dating detrital minerals. If magmatic and metamorphic evolution was continuous in the areas from which sediments moved, then the youngest grain is a good guide to the maximum age of the sediment being analysed. The more strata are analysed in this way the better the detail of sedimentary timing. But two tectonic terrains are unlikely to produce zircons time and time again during a period approaching a billion years. The data indicate only 3 or 4 episodes of ‘zirconogenesis’ in the sedimentary hinterlands, between about 900 to 1940 Ma. Apart from helping correlate sedimentary formations that were previously deemed stratigraphically different – which did help in tectonically unravelling this complex major feature – several hundred isotopic analyses of zircons and micas have give much the same timing as was known already in more precise terms from stratigraphy assisted by a few dozen conventional radiometric dates.