page_title: Sedimentary isostasy
section_title: History of the sedimentary isostasy model
hierarchical_section_title: Sedimentary isostasy / History of the sedimentary isostasy model
caption_attribution_description: English: Grabeau's concept in this diagram was a clear precursor to the chronosome model
context_page_description: Isostasy is a model that describes how upper lithospheric buoyancy adjusts to any significant change in overburden pressure; it responds to significant additional masses of ice or sediment by subsiding. This equilibrium buoyancy model is supported by a hypothetical mobile magma that exists at depths not yet penetrated by the drilling rig.
  Sedimentary isostasy is a related model that addresses how space is created below base level to accommodate additional influxes of sediment. This model assumes that relative sea level remained stable for extensive periods of geological time, despite the continental crust's level changing intermittently throughout the Phanerozoic. The significance of the model is that it can help explain complex geological events observed in sedimentary basins across the globe. In basin analysis it provides a viable alternative to the Sequence stratigraphy model, which implies that sea levels frequently rose above present sea-level and that the crust independently oscillated up and down throughout geological time. It also provides firm theoretical support for the emerging precepts of tectonostratigraphy.
context_section_description: The origin of the sedimentary isostasy model is secured in a letter from John Herschel to Charles Lyell dated 20 February 1836. Herschel was writing from Cape Town, where he was engaged in an astronomical survey of the southern skies. In this letter he discusses the relationship between crustal uplift and submergence using the limited technical vocabulary of his time. The accompanying sketch shows his firm grasp of a viable mechanism to explain the balance between sediment deposition and adjacent tectonic uplift. Unfortunately his insight only achieved publication two years later in Paragraph I of an Appendix to Charles Babbage's broadly philosophical Ninth Bridgewater Treatise Herschel was a polymath; his fundamental contribution to geology was smothered by his immense contributions in other fields and shortly was forgotten in the heat of geological discovery during that decade.
  Charles Babbage (1791–1871) and Charles Lyell (1797–1875) had already been considering possible relationships between granite and stratified rocks. Babbage applied his Analytical engine to the problem of granite compressibility. In 1837 Lyell inserted a frontispiece in the 5th edition of his Principles of Geology that marks a major advance in deep crustal interpretation. Following Herschel's model, it shows a layer at no great depth, where the combination of heat and pressure renders rocks pliable; at greater depths still, they become completely degraded to a mobile granite melt. The lateral flow of melt implied by that diagram provided compelling support for Herschel's concept of sedimentary isostasy.
  
  Homeward bound from his voyage around the world aboard HMS Beagle 1831–1836, Charles Darwin (1809–1882) HMS Beagle called at Cape Town on 3 June 1836 and visited Herschel accompanied by Captain FitzRoy. On return to England, Darwin reported some key evidence observed along the shores of Patagonia 
  'Considering the enormous power of storm waves, it is unlikely that a sedimentary deposit could have survived the ordeal of beach erosion, unless before uplift, it had considerable thickness and a wide extent offshore'. If it were uplifted slowly, the coast should have faced the present ocean with a gentle slope, rather than by a series of steps. This realisation led Darwin to expand the description of sedimentary isostasy: 'Also, on a moderately shallow bottom, where most living marine creatures are found, it seems impossible that a thick expanse of sediment could be deposited widely, unless its bottom sank sufficiently to accommodate successive layers. I am inclined to believe these subsiding movements were accompanied elsewhere by those of elevation'.
  Amanz Gressly (1814–1865) studied strata in the Swiss Jura Mountains and described a vertical succession of groups or terrains there. He then traced beds in horizontal dimensions and encountered marked lateral changes in lithology and paleontology in each stratum. Addressing these lateral changes, he proposed (1838) the term facies and identified sandy, muddy, littoral pelagic and other facies, describing them in terms of both sedimentary type and their inferred environment of deposition.
  
  The next significant step was made by the Russian geologist Nikolai A. Golovkinsky (1834–1897). He formulated his Reverse rule, which states that the boundaries of lithologically and palaeontologically similar layers regularly slide in time when traced laterally. This rule substantiated the primary asynchrony of implied biostratigraphic boundaries. Thus, age-related disposition of the boundaries was associated not only with facies differences in same-aged layers, but also with the palaeobiogeographic features of their faunas. Golovkinsky realized that this contradicted "conventional" views about the process of sedimentation, concluding: 
  the common belief in the sequence of formation of successive layers 'is not true.'
   (Golovkinsky, 1868, p. 125) and remarked that that this would seem paradoxical to many geologists.
  In 1894 Johannes Walther (1860–1937) enunciated the Law of Correlation of Facies, which in essence sta
caption: Regressive off-lap and the formation of a sandstone of emergence shows Grabeau's ideas of regression.
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