Reef geology: studying the sea to understand the mountains

After a longer hiatus a new fieldtrip following the ideas, personalities and outcrops that made the history of geology.

One of the earliest descriptions of coral-reefs comes from a certain Mr. Strachan, who in 1704 submitted to the Royal Society in London a paper about three pages dealing with the subject. He speculated about the formation of this structure, and wrote:

"There are big banks of this coral, it is porous and so hard or yet as smooth as the upright, which grows in small branches. If, of which we speak, is fully grown, others grow in between it, where still others will grow, until the whole structure is as hard as a rock."

This idea of reef-formation was probably not verified in the field, but based on the few travel notions or sparse log book entries brought back from ships venturing in the Indian and Pacific Ocean at these times.
In 1772 to 1775 the German Georg Forster (1729-1798) acted as naturalist on the world-circumnavigation of James Cook, in the years 1773 and 1774 they visited the Pacific Ocean, where Forster studied the atolls and volcanic islands. He observed that corals life in the first meters of the water column, but that the reefs rise up to 300 and 600m above ground. He developed two hypotheses to explain this observation, slow growing the corals raised from the ground, until reaching the surface, where erosion and deposition took place

"They raise their habitation within a little of the surface of the sea, which gradually throes shells, weeds, sand, and bits of corals, and other things o the tops of those coral rocks…[]"

or as alternative, violent volcanic eruptions pushed the corals on the surface.

Fig.1. Atoll as seen in Darwin´s work of 1842: The Structure and Distribution of Coral Reefs.

Darwin during his voyage on board of HMS Beagle (1831-1836) studied Lyell's "Principles of Geology", and the chapter about reefs in the Pacific arouse his interest and his imagination. In Chile, on February 20, 1835, Darwin experienced a very strong earthquake and shortly afterward saw evidence of several meters of uplift in the region, in accordance to Lyell's view Darwin imagined that mountains could rise and sink by many such events during geological time.
Based on the description in the book of atolls, and the possibility of slowly subsidence of earth's surface, and before even seeing a real reef, Darwin developed a preliminary hypothesis to explain the formation of atolls in the middle of the sea, an intriguing geological problem at these times.

"No other work of mine was begun in so deductive a spirit as this; for the whole theory was thought out on the west coast of S. America before I had seen a true coral reef. I had therefore only to verify and extend my views by a careful examination of living reefs. But it should be observed that I had during the two previous years been incessantly attending to the effects on the shores of S. America of the intermittent elevation of the land, together with the denudation and deposition of sediment. This necessarily led me to reflect much on the effects of subsidence, and it was easy to replace in imagination the continued deposition of sediment by the upward growth of coral. To do this was to form my theory of the formation of barrierreefs and atolls."
(Darwin in his autobiography 1887)

Lyell in 1831 could explained fringing-reefs on the coasts of continents by slow subsidence of the landmasses, but didn't extend this idea to the seafloor, Darwin assumed that a single peak of an extinct volcanic mountain, a common feature in the oceans (Darwin had visited various islands of volcanic origin previously), would also experience a slow subsidence, slow enough to enable corals to compensate the down-movement and keep living in the first meters of the water-column, where light and nutrients are available.

Fig.2. Model of atoll formation by reef growth (Darwin, 1842). Darwin proposed that volcanic islands with fringing reefs, islands with barrier reefs and atolls (i.e. ring-shaped reefs without a volcanic island) are different stages of one process, governed by subsidence and reef growth. This famous concept is based on surface examination of reefs and comparison of islands and atolls in different stages of development. Data on the slopes and basins were virtually absent at the time.

Darwin later confirmed his hypothesis by studying reef building organisms and reef morphologies in the Pacific and Indian Ocean (Tahiti in November 1835, Cocos-Keeling Islands and Mauritius in April 1836). He introduced a three-fold classification of reefs that is still used today: fringing reefs, barrier reefs and atolls, also he elaborated a theory that these diverse reefs represent a temporal succession - fringing and barrier reef stages develop around subsiding islands, to became finally an atoll (DARWIN 1837, 1842).
Despite the diligence of Darwin, this work was not comparable with later books, it contains a lot of speculations and superficial observations due the lack of hard evidence, like cores, at these times.

However the theory became soon accepted, Lyell inserted Darwin´s theory in later editions of his "Principles", and the geologist James Dwight Dana (1813-1895), who in 1838-1842 had also visited the Pacific, confirmed most of the observations of Darwin, however recognizing that some areas thought by Darwin to sink didn't in fact show any signs of movements.

Serious critic arouse in the late 1870's, when the German zoologist Carl Semper (1832-1893) in 1868 described on the island of Palau the simultaneous occurrence of the three diverse reef stages, deposing the temporal sequence as seen by Darwin. In 1878 and 1880 the oceanographer John Murray (1841-1914) published his observation made during the Challenger-Expedition (1872-1876) on the islands of Palau and the Fijis. He postulated that reefs grow on submarine elevations of any kind, where organic debris becomes accumulated before dissolving in the seawater.
This new theory was strongly supported and modified by the geologist Alexander Agassiz (son of R. Agassiz) and others. Atolls grew up from shallow submarine summits of various origins, the reef that grows on this bank eventually grows so large that coral the middle of the reef dies, and is dissolved away creating the distinctive shape of an atoll (AGASSIZ 1903).

Darwin responded to Agassiz in 1881, suggesting that the answer could be gained by drilling into an atoll; he assumed that the reef would reach a thickness of 150-180m, below the primary substrate (volcanic rocks for Darwin's hypothesis, or sedimentary rocks for Agassiz's hypothesis) should be found.
In 1896, a Royal Society of London expedition went to the South Pacific to drill beneath Funafuti Atoll in Tuvalu. This expedition was unsuccessful because they were only able to drill down 30m. In 1897 a second expedition drilled down to 210m, and in 1898 they reached a depth of 340m, the outermost limit for the drill-equipment at these times, however still finding calcareous rocks.
In 1951 the U.S. Atomic Energy Commission initialized an extensive drill program to study the structure of the Eniwetok Atoll (Marshall Islands), test site for various nuclear weapons from 1948-1958. This expedition was able to reach a depth of 1,6 kilometres, and finally struck volcanic rock. Darwin had greatly underestimated how thick atolls could be, however it seemed that for the rest he was right.

Even before drilling programs and the use of seismic profiles were available, hinds to understand the structure of atolls came from a very unusual pace, the 2.000m high peaks of the Dolomites in the Alps.
In commission of the Austrian Geological Survey the geologist Baron Ferdinand F. von Richthofen (1833-1905) in 1860 begun to map the area of the valley of Predazzo.
In some stratified sandstones and tuff-deposits he discovered large limestone boulders, some containing fossils and corals, and huge isolated dolomitic massifs emerging from these stratified deposits. Von Richthofen recognized in these particular settings the Darwinian model of atolls - islands of carbonates surrounded by siliciclastic sediments of the ocean-floor, and suggested that the Dolomites in the geological past might have been atolls and barrier reefs such as now found in the South-Western Pacific.


Fig.3. The "Richthofen-Riff", a Triassic reef-compex with clinostratification (from the left to the right) into the basin sediments (St. Kassian-Fm; Wengen-Fm, mainly brown sandstones/shales/marls) by MOJSISOVICS 1879. The hard dolostone stands out from the friable shales and marls covered by meadows.

The naturally occurring cross-section in the Dolomites provided a unique possibility. It was the young geologists Edmund Mojsisovics von Mojsvar (1833-1905) who should developed further the coral-reef hypothesis, mapping the relationships between the single facies, ranging from the lagoon of the atoll to the oceanic basin. Edmund Mojsisovics von Mojsvar was a scion of an expatriated Hungarian family, he studied law, but a strong passion for the Alps led to a professional career with the Austrian Geological Survey in Vienna. Mojsisovics came to the Dolomites after establishing a biostratigraphy of Ammonites in the Austrian Northen Alps, in an attempt to correlate the various Triassic limestone-sequences in the Alps.
The detailed mapping by Mojsisovics, revealed the extent of the facies changes postulated by Richthofen, massive, generally dolomitized limestone reefs in the order of a kilometre thick passing on one side into thinbedded dasyclad-bearing lagoonal carbonates while, on the other side, they wedge out rapidly via steeply inclined foreslope beds of talus breccia into shaly and sandy basinal sediments and basalts, an order of magnitude thinner.

Fig.4. The Langkofel is composed of massive dolostone (reef-facies), suddenly falling off to the basin-sediments in the foreground (stratified St. Kassian-Fm and Wengen-Fm).

Such strong facies changes were until then considered impossible, the reconstruction of an ancient atoll landscape seemed so radical that Mojsisovics was obliged to find a private publisher for his work (MOJSISOVIC 1879).


Fig.5. Examples of slope bedding in the outer parts of the carbonate platforms of the Dolomites (after MOJSISOVIC 1879). Scheme of bedding on the flanks of carbonate platforms and examples of flank and basin deposits from the Sciliar/Schlern platform. Note the abundant limestone boulders in the basin sediments.

Fig.6. Limestone-boulders (former reef-debris), so called Cipit-Kalkblöcke interbedded in the Wengen-Fm. Encapsuled in the impermeable sediments this boulder escaped diagenesis and dolomitization.

Mojsisovics also interpreted the isolated limestone boulders of Richthofen - the so-called "Kalkstein von Cipit" - as gravity-displaced reef material that has slid down the steep outer slopes of the reefs, and became preserved in its original lithology and structure by the tight seal of shale and marl.

Bibliography:

AGASSIZ, A. (1903): Reports on the Scientific Results of the Expedition to the Tropical Pacific, the Marshall Islands. Mem. Mus. Comp. Zool. Harvard College 28: 271-329
DARWIN, C. (1837): On certain areas of elevation and subsidence in the Pacific and Indian oceans, as deduced from the study of coral formations. Proceedings of the Geological Society of London 2: 552-554
DARWIN, C. (1842): The Structure and Distribution of Coral Reefs. D. Appleton & Co., New York: 214
DARWIN, C. (1898): The Structure and Distribution of Coral Reefs. 3th edition, D. Appleton & Co., New York: 214
DARWIN, F. ed. (1887): The life and letters of Charles Darwin, including an autobiographical chapter, Volume 3. John Murray: London.
DOBBS, D. (2005) Reef Madness: Charles Darwin, Alexander Agassiz and the meaning of coral. Pantheon Books: New York
FISCHER, A.G. & GARRISON, R.E. (2009): The role of the Mediterranean region in the development of sedimentary geology: a historical overview. Sedimentology 56: 3-41
MOJSISOVIC, E.v. (1879): Die Dolomit-Riffe von Südtirol und Venetien: Beiträge zur Bildungsgeschichte der Alpen. Alfred Hölder, Vienna: 551
RISTVET, B.L. (1987): Geology and Geohydrology of Enewetak Atoll. In Devaney et al. (ed.), The Natural History of Enewetak AtollVol.1. The Ecosystem; Environments, Biotas, and Processes: U.S: Department of Energy: 37-56
SCHLAGER, W. & KEIM, L. (2009): Carbonate platforms in the Dolomites area of the Southern Alps - historic perspectives on progress in sedimentology. Sedimentology 56: 191-204

Online Resources:

BERGHAUS, H. (1892): Alpenländer geologische Karte nach v. Mojsisovics. Entw. v. Herm. Berghaus 1889, Ausg. 1890. Gest. v. A. Herrmann. Gotha: Justus Perthes (1892). Atlas der Geologie, Berghaus' Physikalischer Atlas. Justus Perthes, Gotha