Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor (Wilson and Palmer, 1992). A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone sequences and distinguished from later-lithified sediments by evidence of exposure to normal marine waters. This evidence can consist of encrusting marine organisms (especially bryozoans, oysters, barnacles, cornulitids, hederelloids, microconchids and crinoids), borings of organisms produced through bioerosion, early 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). Modern hardgrounds are usually detected by sounding in shallow water or through remote sensing techniques like side-scan radar.
Carbonate hardgrounds often host a unique fauna and flora adapted to the hard surface. Organisms usually cement themselves to the substrate and live as sessile filter-feeders (Brett and Liddell, 1982). Some bore into the cemented carbonate to make protective domiciles (borings) for filter-feeding. Sometimes hardgrounds are undermined by currents which remove the soft sediment below them, producing shallow cavities and caves which host a cryptic fauna (Palmer and Fürsich, 1974). The evolution of hardground faunas can be traced through the Phanerozoic, from the Cambrian Period to today (Taylor and Wilson, 2003).
Carbonate hardgrounds were most commonly formed during calcite sea 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 single 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 distinct differences between the Paleozoic and Mesozoic hardground communities: the former are dominated by thick calcitic bryozoans and echinoderms, the latter by 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 as tidal channels (Wilson et al., 2005) and shallow marine carbonate ramps (Palmer and Palmer, 1977; Malpas et al., 2004)
- 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.E.; Liddell, W.D. (1981). "Preservation and paleoecology of a Middle Ordovician hardground community". Paleobiology. 4 (3): 329–348. doi:10.1017/s0094837300006035.
- Fürsich F.T., Kennedy, W.J., Palmer, T.J. (1981). "Trace fossils at a regional discontinuity surface: the Austin/Taylor (Upper Cretaceous) contact in central Texas". Journal of Paleontology. 55: 537–551.CS1 maint: multiple names: authors list (link)
- Fürsich, F.T.; Oschmann, W.; Singh, B.; Jaitly, A.K. (1992). "Hardgrounds, reworked concretion levels and condensed horizons in the Jurassic of western India: their significance for basin analysis". Journal of the Geological Society of London. 149 (3): 313–331. Bibcode:1992JGSoc.149..313F. doi:10.1144/gsjgs.149.3.0313. S2CID 130374753.
- Malpas, J.A.; Gawthorpe, R. L.; Pollard, J.E.; Sharp, I.R. (2004). "Ichnofabric analysis of the shallow marine Nukhul Formation (Miocene), Suez Rift, Egypt: implications for depositional processes and sequence stratigraphic evolution". Palaeogeography, Palaeoclimatology, Palaeoecology. 215 (3–4): 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 fauna". Palaeontology. 17: 507–524.CS1 maint: multiple names: authors list (link)
- Palmer, T.J.; Palmer, C.D. (1977). "Faunal distribution and colonization strategy in a Middle Ordovician hardground community". Lethaia. 10 (3): 179–199. doi:10.1111/j.1502-3931.1977.tb00608.x.
- Palmer, T.J.; Wilson, M.A. (2004). "Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas". Lethaia. 37 (4): 417–427 . doi:10.1080/00241160410002135.
- 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 (4): 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 Late Ordovician (late Mohawkian-Cincinnatian) foreland basin rocks, Kentucky and Virginia". AAPG Bulletin. 81: 1866–1893. doi:10.1306/3b05c654-172a-11d7-8645000102c1865d.
- Taylor, P.D.; Wilson, M.A. (2003). "Palaeoecology and evolution of marine hard substrate communities". Earth-Science Reviews. 62 (1–2): 1–103 . Bibcode:2003ESRv...62....1T. doi:10.1016/S0012-8252(02)00131-9.
- Vinn, O.; Wilson, M.A. (2010). "Microconchid-dominated hardground association from the late Pridoli (Silurian) of Saaremaa, 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): 63–70. doi:10.4267/2042/56744. Retrieved 2015-06-18.
- Wilson, M.A.; Palmer, T.J. (1992). "Hardgrounds and hardground faunas". University of Wales, Aberystwyth, Institute of Earth Studies Publications. 9: 1–131.
- Wilson, M.A.; Wolfe, K.R.; Avni, Y. (2005). "Development of a Jurassic rocky shore complex (Zohar Formation, Makhtesh Qatan, southern Israel)". Israel Journal of Earth Sciences. 54 (3): 171–178 . doi:10.1560/71EQ-CNDF-K3MQ-XYTA.
- Vinn, O. (2015). "Sparsely encrusted hardground in the Darriwilian calcareous sandstone of Cape Pakri, NW Estonia (Baltica)" (PDF). Estonian Journal of Earth Sciences. 64: 249–253. doi:10.3176/earth.2015.31. Retrieved 2015-09-23.
- Vinn, O.; Wilson, M.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 Hard Substrates in the Ordovician of Estonia (Baltica)". PLOS ONE. 10 (7): e0134279. Bibcode:2015PLoSO..1034279V. doi:10.1371/journal.pone.0134279. PMC 4517899. 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 Journal of Earth Sciences. 65: 19–26. doi:10.3176/earth.2016.01. Retrieved 2016-03-05.