Hardground Resources

Hard Facts About Hardgrounds
Taxonomic Survey of Known Marine Borers
Petrology of Carbonate Hardgrounds
Taphonomy of Trace Fossils at Omission Surfaces [PDF]

Dan Wonderly on Hardgrounds: Excerpts from 'Neglect of Geologic Data,' p. 24-27

In many of the carbonate rock layers of the world we find 'hardground surfaces.' In such cases the layers of rock have visible characteristics on their upper surfaces which show that each such surface was exposed to at least some scouring, dissolution, or other alteration after it was lithified and before the succeeding layer of limestone was added above it.

Since these hardground layers are marine in origin, many of them have an abundance of marine fossil shells embedded in the limestone. Commonly, in such a layer, the shells which are at the upper surface are extensively eroded (truncated) so that only one half or less of the shell remains -- solidly embedded in the hard limestone. Since this rock layer was lying in water while the erosion was going on, encrusting-type, lime-secreting, marine animals (such as oysters) are frequently found on the eroded surfaces. Also many of the eroded surfaces have been 'bored' by sponges and other types of marine animals which bore holes in the rock by a process in which they secrete acid which dissolves the carbonate rock. The inner surfaces of these 'bored' holes frequently contain the truncated remainder of compoenent grains of the rock which where cleanly cut off by the animal as it advanced deeper into the rock (Bathurst, 1975, pp. 373 and 395-397; Wilkinson, et al., 1985, pp. 171-173).

It is very evident that all of these processes of change in the upper surface of the layer required several years of time. And on must not forget that an extended period of time wa required for cementation of the carbonate grains into the form of a hard layer before thees processes of erosion, encrusting, and boring could begin. . .

Some of the best examples of older carbonate hardgrounds which have been formed in ancient rock systems on the continents are the following. (1) a formation of Jurassic limestones in Lorraine in France containing 30 to 40 hardgrounds, with many encrusting and boring organisms represented (Jaanusson, 1961, p. 228; compare Bathurst, 1975, p. 396, Fursich, 1979m p. 27, and Purser, 1969). Fursich (1979, pp. 3-9) lists pver 30 locations in Europe where Jurassic hardgrounds are located, and gives references for the descriptions of them. (2) A Devonian formation in Russia in which hardgrounds with 'a rich epifauna . . . occur at many different levels' (Jaanusson, 1961, p. 227). (3) An Ordovician formation in Sweden, slightly over 6 meters thick, containing a succession of fossiliferous hardgrounds, with the beds being from 2 to 20 cm thick, with marl of shale between them (Bathurst, 1975, pp. 397-399). (4) Hardgrounds of Middle Ordovician limestone sequences in southwest Virginia (Read and Grover, 1977, p. 961-963). These exhibit encrustation by bottom-dwelling marine organisms such as bryozoans. Also most of the hardground surfacers are impregnated by brown to black opaque minerals which obviously accumulated on them by precipitation before they were covered over by the next succeeding layer. . . (5) An excellent example of a Jurassic carbonate hardground sequence here in the United States which has been carefully described and studied in detail is that found in the Sundance Formation of southeastern Wyoming . . . A thorough study of the petrology, inferred depositional environments, and cement types of this formation has recently been made by Wilkinson, Smith, and Lohmann (1985). This latter study carefully investigated the cement crystals which were formed during early lithification (i.e. before erosion and burial) of at least 9 of the hardgrounds in the vertical sequence found in this part of Wyoming. One particular layer of these hardgrounds was identified at several widely-seperated sites covering an area of approximately three thousands square miles.

The hardgrounds in Wyoming are composed mainly of identifiable biogenic particles and are an unusually good example of how such layers were bored and encrusted after having been well lithified by early cement. Many of the borings made clean-cut truncations, not only of the well-formed grains of the early carbonate cement, but also of the ooids and fragments of fossils which had been cemented into the layer. . . After burial, pore-filling carbonate cement filled in most of the holes and other cavities which has been made by boring endolithic fauna and also formed 'late cement' layers over many of the shells of animals which has encrusted the hardgrond layers. . . Wilkinson and his colleagues summarize these events as follows:

Macroscopic features demonstrate that sandstone and limestone units were repeatedly lithified during deposition of the Sundance Formation. Boring by endolithic mollusks as well as encrustation by oysters and serpulid worms requires the formation of well-lithified substrates prior to deposition of overlying units. Rounding of sandstone and limestone clasts further attests to the repeated development of well-indurated units during deposition (p. 179).

 

Letter from Mark Wilson to Answers in Genesis

(See also Tas Walker's laughable rebuttal. It is a a true gem of pseudogeology. I especially like the "squeezed plum" explanation of marine borings!)

To the Editorial Team at AiG:

The following is designed to be published in your “feedback” section. As you will see soon, it is “negative”. This letter has not been submitted elsewhere. I agree to transfer copyright to Answers in Genesis. You may use my full name and address when the letter is posted on your website.

I am a geologist at The College of Wooster in Ohio. I have long followed the creation/evolution debate, and I am very familiar with Answers in Genesis and other creationist organizations. I respect your courage and enthusiasm, but I believe you are very wrong about evolution, the fossil record, and the age of the Earth.

Part of my geological research involves the development of carbonate hardgrounds and their associated fossil communities in the sedimentary record. Hardgrounds and their fossils show that many limestones formed over long intervals of time, at least hundreds if not thousands of years. I believe, of course, that the sedimentary rock column was formed over hundreds of millions of years, but with this letter I simply want to demonstrate that many rocks could not possibly have formed during the few days or months allocated to them in your ”flood geology” model. I have supported my comments below with citations of work I have done so that there can be no confusion as to the nature of the evidence.

A carbonate hardground is a synsedimentarily-cemented carbonate seafloor (see, for examples and further references, Lethaia 25:19–34, 1992, and Geology 26:379–381, 1998). Under certain conditions on the floor of a shallow tropical sea, usually a reduction in sedimentation rate, increased circulation of seawater through pore space, and elevated carbon dioxide levels, calcium carbonate will crystallize between sedimentary grains and cement them together. In a real sense, then, this process is the lithification of the sediment on the seafloor prior to burial. Forming a hardground probably takes years, but we do not yet have any direct way to estimate the duration of the process.

After the sediment is lithified into a hardground, many animals and plants colonize this hard substrate. They have particular adaptations for living on hard surfaces. Many bryozoans, for example, produce a modular calcitic skeleton which adheres tightly to the surface and grows laterally and, often, upward into mounds or stout branches. Some crinoids attach themselves with special disks (holdfasts) which allow them to extend their long crown-topped stalks into food-bearing currents above the hardground.

Oysters cement themselves onto the substrate, as do other organisms such as cornulitids, barnacles, sponges and algae. Many of these creatures have easily-fossilized skeletons, so they are often found encrusting ancient hardgrounds (see, for examples, Palaeontology 42:887–895, 1999; Ichnos 3:79–87, 1994; Journal of Paleontology 67:1011–1016, 1993; Nature 335:809–810, 1988).

In addition to these encrusting organisms, many other animals drill holes directly into hardgrounds and other hard substrates to build dwelling spaces. These boring organisms include bivalves, sponges, barnacles and worms of various sorts. Most use a combination of chemicals (such as acids) and skeletal devices to excavate their holes. Many of these borings show the growth of the drilling animal as the hole was deepened (see Journal of Paleontology 72:769–772, 1998; Palaios 13:70–78, 1998). Bivalves, for example, drill holes which start with a small diameter for the juvenile bivalve, and then they increase in diameter with depth as the bivalve grows. The result is a hole with a bivalve in it far too large to get out the restricted opening. This is not a problem, of course, because the animal is a filter-feeder with no need to leave its rocky home. Hardgrounds and other hard substrates are often riddled with several generations of these borings (see Palaeontology 29:691–703, 1986).

The problem for creationists comes when these carbonate hardgrounds and their faunas are forced into a “flood geology” framework where they must have formed in just a few days or months. Fossiliferous hardgrounds are found throughout the geological record, from the Cambrian through the Recent, so they are not restricted to any particular level (The Paleontological Society Special Publication 5:137–152, 1990). A hardground community is preserved in place (not transported) because the organisms either encrusted the substrate or had bored into it. The encrusters had to land as larvae on the hardground after it was lithified, and then they developed into adults. It is clear from the size and distribution of many of these fossils that this development took years and was often halted and restarted by physical disturbances (see Science 228:575–577, 1985). How could this have happened many times during the course of a massive, short-lived flood?

In addition, the boring animals also show the length of time required for these hardground communities to mature. The borings themselves were formed during the lifespans of individuals, which was usually years. The borings often cut into other borings from previous generations, many of which are filled with cemented sediment. The borings are sometimes overgrown by encrusters, again showing the depth of time available for this community development (see Palaios 13:70–78, 1988, for a good example). Again, this biological process happened dozens and dozens of times through the sedimentary rock record. Again, this cannot be explained within the flood model, but it is entirely consistent with a rock record hundreds of millions of years old.

I am anxious to see how you respond to this challenge from hardgrounds and their fossils. I cannot conceive of a creationist model which can account for them in the context of a single global flood. They are instead one of many indicators of an Earth millions of years old with a history long preceding that of humanity.

Journal Sedimentary Research. 69(1) January 1999.

QUANTIFYING RATES OF SYNDEPOSITIONAL MARINE CEMENTATION IN DEEPER PLATFORM ENVIRONMENTS---NEW INSIGHT INTO A FUNDAMENTAL PROCESS

G. MICHAEL GRAMMER, CHRISTOPHER M. CRESCINI, DONALD F. MCNEILL, AND LOUIS H. TAYLOR

Figure 5

ABSTRACT: Syndepositional marine cementation on carbonate platforms is a fundamental process that results in physical stabilization and lithification of sediments, as well as reduction in primary porosity and permeability which may affect the migration of later diagenetic fluids. Rapid submarine cementation is commonly invoked to explain various aspects of carbonate platform geometry, especially related to the preservation of steep slopes, but the rates typically reported for cementation in deeper parts of the platform are mostly conjectural. In an attempt to quantify the rates of syndepositional cementation in marginal platform environments, substrates containing samples of carbonate sand (ooids) suspended above the sea floor were emplaced at various depths across a modern platform in the Bahamas to evaluate the rates of cement growth through direct observation. Initial results indicate that cementation may lead to partial lithification of carbonate sands within a minimum of 8 months in water depths of up to 60 m. These results provide some of the first direct evidence for rates at which cementation can occur in deeper marginal environments, and confirms earlier hypotheses that very rapid, syndepositional marine cementation may not be limited to very shallow subtidal and intertidal environments.

RUSSO, F., NERI, C., MASTANDREA, A. and BARACCA, A. (1997): The Mud Mound Nature of the Cassian Platform Margins of the Dolomites. A Case History: the Cipit Boulders from Punta Grohmann (Sasso Piatto Massif, Northern Italy)

Facies 36, 25-36, Pl. 8-10, Erlangen 1997

Summary: The sedimentological features and the microbiofacies of the Cassian platforms (Late Ladinian-Carnian) of the Dolomites can be studied only on the basis of the so-called "Cipit boulders", that are platform-derived olistoliths and clasts fed to the basin and escaped to the extensive dolomitization affecting the buildups.
Our paper deals with the Cipit boulders occurring in the Punta Grohmann section (Wengen and S. Cassiano formations, Late Ladinian, Archelaus and Regoledanus Zones). The dominant microfacies are represented by boundstone, consisting of nearly 60% of micritic limestone occurring both as peloidal or aphanitic micrite, mostly organized into stromatolitic laminites or thrombolites. The skeletal organism (Tubiphytes, skeletal cyanobacteria, sphinctozoan sponges, etc.) represent only a minor component of the rock (usually less than 10%). Early cements are widespread and consist both of fan-shaped calcite (replacing former aragonite), bladed isopachous magnesian calcite and radial-fibrous calcite (neomorphic after Mg-calcite). The carbonate platforms from which the olistoliths derive were made up mainly of carbonate mud that underwent early lithification, as witnessed by the considerable amount of early cements: therefore they may be regarded to as mud-mounds, and more precisely as microbial mud-mounds, due to the clearly accretionary, organic-controlled nature of most micrites. The micrites, subdivided into auto- and allomicrite on the basis of micromorphological and fabric characteristics, have been tested for epifluorescence. The results confirm the organic control on the deposition of automicrite, also in the cases in which a microbial influence is not obvious (i.e., aphanitic micrite without internal organization).

FORMATION AND DIAGENESIS OF A COOL-WATER HARDGROUND IN THE OLIGOCENE NILE GROUP, WESTLAND, NEW ZEALAND

Scholle, P. A., and Lewis, D. W.

American Association of Petroleum Geologists, 1999 Annual Convention Program, v. 8, p. A125.

Abstract: A hiatal surface in the Nile Group provides an example of hardground formation on a narrow and steep, tectonically active, shelf-slope area in a cool-water setting. The sediment contains bivalves, bryozoans and brachiopods in a micritic matrix. Seafloor cementation, followed by extensive boring of the hardened sediment, provided pathways for shallow sub-seafloor dissolution of originally aragonitic bivalves without collapse of surrounding matrix. Bivalve molds were then filled with infiltrated or authigenic sediment--detrital terrigenous quartz and feldspar grains and peloidal glauconite (materials that are scarce in surrounding matrix), plus micritic and peloidal carbonate sediment. Carbonate mold-fills were later replaced by equant, medium-crystalline dolomite. Most of that dolomite, in turn, was altered to a "rusty" dedolomite (ferroan calcite after dolomite). This combination of noted features leads to the following conclusions: 1) initial diagenesis had to be essentially penecontemporaneous; 2) a slow-down or cessation of carbonate production led to early lithification, presumably by Mg-calcite cement, and allowed time for formation of glauconite and accumulation of terrigenous sediment; 3) dolomitization of mold-filling sediment apparently also occurred in a near-seafloor setting, probably as a result of prolonged contact with seawater, as determined from stable isotope geochemistry; 4) subsequent burial and uplift of these strata led to formation of additional calcite cements and near-surface alteration of dolomite to calcite.

The Nile Group hardgrounds differ in many respects from those found in other, broader shelf settings of similar age in the Tertiary of New Zealand--they have far less phosphate and far more dolomite and are much less regionally extensive. Not enough is known about these sediments yet to fully explain such differences.

ISOTOPIC AND PETROPHYSICAL DATA ON HARDGROUNDS FROM UPPER CRETACEOUS CHALKS FROM WESTERN EUROPE [abs.]

Scholle, P. A. and Kennedy, W. J.

Geological Society of America, Abstracts with Programs, v. 6 (7), p. 943, 1974.

Abstract: Hardgrounds are surfaces of synsedimentary submarine lithification recognized by hardness, superficial mineralization, encrusting epifauna, borings, reworked pebbles, and lack of compaction features. They characterize many hiatus intervals in European chalks and individual surfaces can be traced for hundreds to thousands of square kilometers.

Porosity measurements on hardgrounds and associated non-hardground chalks provide minimum estimates of the degree of cementation involved in hardground formation. In chalks with between 30 and 50% porosity, the associated hardgrounds have only 10 to 15% pore space. The original foram-nannofossil oozes near the sediment-water interface, had porosities of ca. 60 to 80% at the time of cementation. Thus, addition of 45 to 70% cement was involved in hardground formation; direct examination of hardground fabrics often shows very large amounts of cement and poor grain orientation due to lack of compaction.

Although bulk carbon and oxygen isotopic values of most European chalks have been considerably altered by diagenesis, isotopic values of associated hardgrounds invariably are less altered (more marine values). Analysed hardgrounds have del O¹⁸ ratios of -1.5 to -2.8 per mil (relative to PDB), while associated chalks range from -2.6 to -7.0 per mil. Carbon isotope ratios of hard.grounds are slightly less positive than in associated chalks. These data imply that cementation of hardgrounds took place in a marine setting, not a subaerial one. Also, the del O¹⁸ ratios show most of the cement to be early, isotopically heavy marine calcite, rather than light, later diagenetic calcite as in associated chalks. Thus, isotopic analysis can be used to conclusively demonstrate synsedimentary submarine lithification of hardgrounds.

Origin of Early to Middle Jurassic Hardgrounds from Condensed Carbonate Sequences in se Spain

Examined hardgrounds located in SE Spain, about 1 km east from the village La Romana, in the vicinity of Cerro the la Cruz (Sierra de Reclot), 35 km west from Alicante. The whole region is situated within eastern part of the External Subbetic which is a part of the External Zones of the Betic Cordillera. The Jurassic sedimentary sequence includes the following formation: Gavilan Fm., Zegri Fm. and Upper Ammonitico Rosso Fm. Sediments are mainly limestones with admixture of marl, occasionally forming nodular limestones. The investigated hardgrounds occur in the top of Gavilan Fm. dated as late Pliensbachian and in the top of Zegri Fm. extended diachronously from Middle Toarcian up to Aalenian. Combined isotopic and elemental analyses have been used in studies of the both hardgrounds. The oxygen and carbon isotope results show very consistent values in the range from -1 to -2 per mil, and around +2 per mil respectively throught the whole sequence except the hardground surfaces. These surfaces are characterised by depleted isotope values. Interestingly, the carbon isotope values are more depleted than oxygen ones. This all suggestes that the reason of calcium carbonate non deposition could not be explained by the influx of fresh waters. Rather volcanic and hydrothermal activity, which might provide the seawater carbonate system with excess of carbon dioxide, could stop carbonate precipitation and created the hardground. The intensitivity of the hydrothermal and volcanic processes could not be very high because the isotopic pertubations is observed only at the very hardground surfaces. The possibility of submarine volcanic activity has been suggested elsewhere (Jimenes-Espinosa et al., 1997). Additionally, the detailed isotopic examination of originally preserved belemnite rostra show very consistent values throught. Calculating the temperature from the oxygen isotope values, the average of 17 degrees Celcius for the mean seawater has been obtained.

Jimenez-Espinosa R, Jimenez-Millan J & Nieto L, Sedim. Geol., 114, 97-107

Journal Sedimentary Research Volume 67(3) May 1997

SUBMARINE CEMENTATION AND SUBAERIAL EXPOSURE IN OLIGO-MIOCENE TEMPERATE CARBONATES, TORQUAY BASIN, AUSTRALIA

STELIOS NICOLAIDES AND MALCOLM W. WALLACE

ABSTRACT: At least four hardgrounds are present throughout the 27-m-thick exposed, Oligo-Miocene, cool-water Point Addis Limestone of the Torquay Basin in southeastern Australia. They range from thin (a few centimeters thick), incipient layers that have a nodular fabric to thick (up to 1 m thick) continuous hardgrounds. The hardgrounds tend to be at the top of coarsening-upward cycles and are typically overlain by low-relief erosion surfaces. The uppermost hardground is the best developed and most widespread in the Point Addis Limestone, and can be observed in all exposures of the succession. At some localities, it shows evidence of karstification such as dissolution pits, clints and grikes, and dissolution pinnacles. Whether karstified or not, well-developed hardgrounds are overlain by a lag conglomerate that consists of hardground intraclasts with marine Fe-oxide crusts. The hardgrounds are cemented by a first-generation isopachous, inclusion-rich, columnar and fibrous radiaxial calcite of marine origin with trace-element compositions (Mg2+ = 0.68--1.74 mole % MgCO3; Fe2+ = 800--4690 ppm; Sr2+ = 0--260 ppm; Mn2+ = 0--230 ppm) and cathodoluminescence (dull/blotchy) indicative of stabilized Mg-calcite. The isopachous cements are invariably overlain by homogeneous, peloidal, or microbioclastic micrite, having trace-element compositions (Mg2+ = 0.77--1.54 mole % MgCO3; Fe2+ = 760--10,030 ppm; Sr2+ = 0--240 ppm; Mn2+ = 60--340 ppm) and cathodoluminescence (dull/blotchy) again indicative of stabilized Mg-calcite. This micrite is always closely associated with the isopachous cements and appears to be of marine origin, perhaps being analogous to the micritic precipitates described from reefal settings. Clear calcite is the last cement generation in the hardgrounds and has attributes of meteoric cements (nonluminescent; Mg2+ = 0.12--0.87 mole % MgCO3; Fe2+ = 0--230 ppm; Sr2+ and Mn2+ below detection limits). The whole-rock stable-isotope compositions of the hardgrounds and host limestone indicate that the whole unit has been subjected to pervasive alteration by meteoric fluids. The least altered carbonates analyzed from the Point Addis Limestone are brachiopods (d13C = -1.5‰ to +2.2‰ PDB; d18O = -1.7‰ to +0.9‰ PDB) and the unkarstified hardgrounds (d13C = -3.8‰ to -1.0‰ PDB; d18O = -0.6‰ to +0.6‰ PDB).

These hardground occurrences help promote the validity of a unique sea-level-driven model for the formation of hardgrounds in cool-water settings. We propose that the development of these hardgrounds was an entirely marine process, produced by relative sea-level drop and entry of the sea floor into the zone of wave reworking. Marine cementation may begin in the form of nodules, at and below the seawater/sediment interface, and involves both slow sedimentation and shallowing of the cool waters. The nodules can later merge to form continuous hardgrounds. In such a high-energy environment, nondeposition and erosion are the dominant processes and marine cementation can occur. Further sea-level drop would lead to subaerial exposure of the previously formed hardgrounds, as found in one instance in the Point Addis Limestone at coastal exposures.