How to Make Hardgrounds Soft Again

Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor (Wilson and Palmer, 1992). A hardground is substantially, and then, a lithified seafloor. Ancient hardgrounds are found in limestone sequences and distinguished from afterward-lithified sediments by evidence of exposure to normal marine waters. This evidence can consist of encrusting marine organisms (specially bryozoans, oysters, barnacles, cornulitids, hederelloids, microconchids and crinoids), borings of organisms produced through bioerosion, early on marine calcite cements, or extensive surfaces mineralized by iron oxides or calcium phosphates (Palmer, 1982; Bodenbender et al., 1989; Vinn and Wilson, 2010; Vinn and Toom, 2015). Mod hardgrounds are commonly detected by sounding in shallow water or through remote sensing techniques like side-scan radar.

Carbonate hardgrounds ofttimes host a unique fauna and flora adjusted to the hard surface. Organisms normally cement themselves to the substrate and live as sessile filter-feeders (Brett and Liddell, 1982). Some bore into the cemented carbonate to brand protective domiciles (borings) for filter-feeding. Sometimes hardgrounds are undermined by currents which remove the soft sediment beneath them, producing shallow cavities and caves which host a ambiguous beast (Palmer and Fürsich, 1974). The development of hardground faunas can exist traced through the Phanerozoic, from the Cambrian Period to today (Taylor and Wilson, 2003).

Heart Jurassic hardground (Carmel Formation) with encrusting oysters and borings.

Scientific papers on hardgrounds by period. Serves as a proxy for hardground abundance over time. Aragonite and calcite bounding main intervals are plotted on the time axis.

Carbonate hardgrounds were most commonly formed during calcite body of water intervals in Earth history, which were times of rapid precipitation of low-magnesium calcite and the dissolution of skeletal aragonite (Palmer and Wilson, 2004). The Ordovician-Silurian and the Jurassic-Cretaceous Systems have the most hardgrounds (sometimes hundreds in a unmarried section) and the Permian-Triassic Systems have the least (usually none). This cyclicity in hardground formation is reflected in the evolution of hardground-dwelling communities. There are singled-out differences between the Paleozoic and Mesozoic hardground communities: the former are dominated by thick calcitic bryozoans and echinoderms, the latter past oysters and deep bivalve (Gastrochaenolites) and sponge (Entobia) borings (Taylor and Wilson, 2003).

Stratigraphers and sedimentologists often use hardgrounds as marker horizons and as indicators of sedimentary hiatuses and flooding events (Fürsich et al., 1981, 1992; Pope and Read, 1997). Hardgrounds and their faunas can also represent very specific depositional environments such equally tidal channels (Wilson et al., 2005) and shallow marine carbonate ramps (Palmer and Palmer, 1977; Malpas et al., 2004)

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  Hardground in the Liberty Formation (Upper Ordovician) of southern Ohio.

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  Cross-section of an Upper Ordovician hardground from Kentucky. The light-colored vertical elements are borings (Trypanites) filled with dolomite. The scale bar is 1.0 cm.

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  A Middle Ordovician hardground from the Kanosh Formation of Utah with echinoderm holdfasts cemented to its upper surface. The scale bar is ane.0 cm.

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  Cross-section of a carbonate hardground encrusted by oysters and bored by bivalves (Gastrochaenolites); Carmel Formation (Middle Jurassic) of southern Utah.

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  Trypanites borings in an Upper Ordovician hardground from northern Kentucky.

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  Carbonate hardground; Ora Germination, Upper Cretaceous (Turonian), southern State of israel.

References [edit]

• Bodenbender, B.E.; Wilson, M.A.; Palmer, T.J. (1989). "Paleoecology of Sphenothallus on an Upper Ordovician hardground". Lethaia. 22 (2): 217–225. doi:10.1111/j.1502-3931.1989.tb01685.x.
• Brett, C.Eastward.; Liddell, W.D. (1981). "Preservation and paleoecology of a Middle Ordovician hardground community". Paleobiology. 4 (3): 329–348. doi:ten.1017/s0094837300006035.
• Fürsich F.T., Kennedy, W.J., Palmer, T.J. (1981). "Trace fossils at a regional aperture surface: the Austin/Taylor (Upper Cretaceous) contact in central Texas". Journal of Paleontology. 55: 537–551. {{cite journal}}: CS1 maint: multiple names: authors list (link)
• Fürsich, F.T.; Oschmann, W.; Singh, B.; Jaitly, A.G. (1992). "Hardgrounds, reworked concretion levels and condensed horizons in the Jurassic of western India: their significance for basin analysis". Journal of the Geological Guild of London. 149 (three): 313–331. Bibcode:1992JGSoc.149..313F. doi:10.1144/gsjgs.149.3.0313. S2CID 130374753.
• Malpas, J.A.; Gawthorpe, R. L.; Pollard, J.Eastward.; Sharp, I.R. (2004). "Ichnofabric analysis of the shallow marine Nukhul Formation (Miocene), Suez Rift, Arab republic of egypt: implications for depositional processes and sequence stratigraphic evolution". Palaeogeography, Palaeoclimatology, Palaeoecology. 215 (iii–4): 239–264. doi:10.1016/j.palaeo.2004.09.007.
• Palmer, T.J. Fürsich, F.T. (1974). "The ecology of a Middle Jurassic hardground and crevice beast". Palaeontology. 17: 507–524. {{cite periodical}}: CS1 maint: multiple names: authors list (link)
• Palmer, T.J.; Palmer, C.D. (1977). "Faunal distribution and colonization strategy in a Heart Ordovician hardground customs". Lethaia. x (three): 179–199. doi:10.1111/j.1502-3931.1977.tb00608.x.
• Palmer, T.J.; Wilson, Yard.A. (2004). "Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas". Lethaia. 37 (4): 417–427 [ane]. doi:x.1080/00241160410002135.
• Palmer, T.J. (1978). "Burrows at certain omission surfaces in the Middle Ordovician of the Upper Mississippi Valley". Journal of Paleontology. 52: 109–117.
• Palmer, T.J. (1982). "Cambrian to Cretaceous changes in hardground communities". Lethaia. 15 (four): 309–323. doi:10.1111/j.1502-3931.1982.tb01696.x.
• Pope, M.C.; Read, J.F. (1997). "High-resolution surface and subsurface sequence stratigraphy of the Middle to Belatedly Ordovician (late Mohawkian-Cincinnatian) foreland bowl rocks, Kentucky and Virginia". AAPG Bulletin. 81: 1866–1893. doi:10.1306/3b05c654-172a-11d7-8645000102c1865d.
• Taylor, P.D.; Wilson, Grand.A. (2003). "Palaeoecology and development of marine hard substrate communities". Earth-Science Reviews. 62 (1–ii): 1–103 [2]. Bibcode:2003ESRv...62....1T. doi:10.1016/S0012-8252(02)00131-9.
• Vinn, O.; Wilson, Thou.A. (2010). "Microconchid-dominated hardground association from the late Pridoli (Silurian) of Saaremaa, Republic of estonia". Palaeontologia Electronica. 2010 (2): 13.2.9A. Retrieved 2012-09-16 .
• Vinn, O.; Toom, U. (2015). "Some encrusted hardgrounds from the Ordovician of Estonia (Baltica)". Carnets de Géologie. 15 (7): 63–seventy. doi:10.4267/2042/56744 . Retrieved 2015-06-18 .
• Wilson, M.A.; Palmer, T.J. (1992). "Hardgrounds and hardground faunas". University of Wales, Aberystwyth, Establish of Earth Studies Publications. 9: ane–131.
• Wilson, M.A.; Wolfe, K.R.; Avni, Y. (2005). "Development of a Jurassic rocky shore complex (Zohar Formation, Makhtesh Qatan, southern Israel)". State of israel Journal of Earth Sciences. 54 (3): 171–178 [3]. doi:10.1560/71EQ-CNDF-K3MQ-XYTA.

Further reading [edit]

• Vinn, O. (2015). "Sparsely encrusted hardground in the Darriwilian calcareous sandstone of Cape Pakri, NW Estonia (Baltica)" (PDF). Estonian Periodical of Earth Sciences. 64: 249–253. doi:10.3176/earth.2015.31 . Retrieved 2015-09-23 .
• Vinn, O.; Wilson, K.A. (2010). "Early large borings from a hardground of Floian-Dapingian age (Early and Middle Ordovician) in northeastern Estonia (Baltica)". Carnets de Géologie. 2010: CG2010_L04. doi:10.4267/2042/35594.
• Vinn, O.; Wilson, M.A.; Toom, U. (2015). "Bioerosion of Inorganic Difficult Substrates in the Ordovician of Estonia (Baltica)". PLOS ONE. x (7): e0134279. Bibcode:2015PLoSO..1034279V. doi:ten.1371/periodical.pone.0134279. PMC4517899. PMID 26218582.
• Vinn, O.; Toom, U. (2016). "A sparsely encrusted hardground with abundant Trypanites borings from the Llandovery of the Velise River, western Estonia (Baltica)" (PDF). Estonian Periodical of Earth Sciences. 65: 19–26. doi:10.3176/earth.2016.01 . Retrieved 2016-03-05 .

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Source: https://en.wikipedia.org/wiki/Carbonate_hardgrounds

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