Field Study of Purported Hardgrounds of the Cincinnatian (Ohio, USA)
Total Page：16
File Type：pdf, Size：1020Kb
Countering the critics Field study of purported hardgrounds — Woodmorappe and Whitmore this report is limited to our recent fieldwork itself. The Field study reconciliation of apparent hardground phenomena with the Universal Deluge is to be presented subsequently.4 Throughout the present work, standard uniformitarian terms of purported related to paleoenvironments and deep time are employed hardgrounds of without in any way implying our acceptance of them. the Cincinnatian Geology of site The Cincinnatian (Upper Ordovician: Caradocian to Ashgillian) strata of the east-central USA is especially (Ohio, USA) known for its abundance of fossils. The online article by Holland provides excellent background information of the John Woodmorappe and John Whitmore local geology.5 The strata consist of a series of alternating shales and limestones. In terms of sequence stratigraphy, A number of samples conventionally believed to the Cincinnatian is divided into five limestone–shale marine be indicators of hardgrounds are located in the transgressive-highstand cycles. The Ashgillian Liberty and Cincinnatian (Upper Ordovician) carbonates of the Whitewater Formations at the Caesar’s Creek Spillway east-central USA. These consist of borings (espe- (39°48'26"N, 84°03'27"W), south-west Ohio, the site of this cially Trypanites) in both fossil and limestone sur- field investigation, are assigned to the highstand part of the faces, as well as some encrusting bryozoans. The fourth depositional sequence.6 The Spillway itself is located ostensible evidence for protracted sedimentation approximately 50 miles (80 km) NNE of the Answers in (hardgrounds) must be juxtaposed against a much Genesis USA headquarters in Petersburg, Kentucky. Over a more commonly occurring set of evidences point- dozen hardground horizons ing to rapid sedimentation. The latter include storm are claimed to have been deposits, cross-bedded carbonates, large ripple located at the Spillway itself marks and a large suite of orthocone nautiloids that by previous investigators,7 are crowded into a single bed. as well as others throughout the Cincinnatian. To Hardgrounds are lithified regions of the sea floor. make matters even more Their alleged ancient counterparts are inferred from the ostensibly challenging for presence of trace fossils (borings) that can only be made on Flood geology, a 1.6-km- a hard substrate, and body fossils of specialized organisms deep borehole just a few believed to require a hard substrate to encrust. A set of kilometres north of the 1 slides depicting inferred hardgrounds is available online. Spillway intersects trilobite- Not surprisingly, hardgrounds have long been presented as bearing Cambrian strata,8 2 a challenge to Flood geology, owing to the long time for implying the transpiration of modern hardgrounds to form. However, they also present a series of time-consuming Figure 2. An in situ bored coral 3 difficulties for conventional uniformitarian geology. Owing events before even the first Grewingkia, partly obscured by to the volume of information relevant to hardgrounds, powdered talus. It opens to the supposed hardground. left, with relatively wide Trypanites boreholes. Penny for scale. Examination of site Field guidebooks9,10 descriptive of the site were studied. Compilations of photos of Cincinnatian fossils,11–13 along with the PowerPoint program developed by Whitmore,14 proved especially useful in the identification of fossils. Osgood’s classic15 was consulted for the identification of trace fossils. Owing to the fact that neither of us are specialists in any relevant fossil group, our identifications are necessarily tentative. Many different kinds of body and trace fossils were seen and collected, but only those relevant to hardgrounds are discussed in this report. Six days of collaborative field work were carried out by the authors in 1999 and 2004. Thousands of square Figure 1. A detailed closeup of a bored coral Grewingkia. metres of talus and outcrop surfaces were examined. In 82 TJ 18(3) 2004 Field study of purported hardgrounds — Woodmorappe and Whitmore Countering the critics Figure 3. A relatively large specimen of a bored coral Grewingkia. Figure 4. An extensively bored specimen of the coral addition, Dr Whitmore, who lives in the area, has done a Grewingkia embedded in a brachiopod coquina. Penny considerable amount of fieldwork at this location, alone for scale. and with his college geology students, over the last 25 years. In the present fieldwork, Woodmorappe focused on the hardground-related evidence, while Whitmore concentrated on the ripple marks, cross-bedded lithologies and the nautiloid bed.16 Owing to the fact that the large Caesar Creek Spillway outcrop is almost entirely vertical with virtually no terraces, and in situ carbonate beds are usually obscured by the remnants of weathered shales, opportunities for seeing fossils and structures in situ are limited. For this reason, and in common with previous investigators, most of our time in the field was spent examining the thick layers of talus that occur at the foot of the outcrop.17 For scale in photos (figs. 1–24), the standard GSA (Geological Society of America) scale in cm was used, as were US coins (quarter—2.3 cm diameter and 1 mm thickness; dime—1.75 cm diameter; penny—1.9 cm diameter). Owing to the fact that the Spillway walls are constantly being eroded, no map exists of the Spillway itself, and none is provided in this report. However, GPS (Global Positioning Satellite) data is included for precise locality information as needed. Figure 5. Sectioned sample showing bedding, burrows, etc., within the strata in which is embedded a bored coral Grewingkia. How ancient hardgrounds are recognized originally hard character of the surface … . Most In order to identify hardgrounds in the sedimentary hardgrounds go unrecognized in the field because record, one has to recognize body fossils and trace fossils the fossils associated with them are not usually no- of organisms that ostensibly required a hard seafloor ticed. We hope to increase the number of recorded substrate to colonize. In terms of specifics, the presumed hardgrounds by improving the abilities of geologists recognition of hardgrounds in the sedimentary record can to distinguish the often esoteric fossils associated be summarized in the following quotation: with them … . Since borings are the last faunal ‘Although many hardgrounds show a charac- features to succumb to erosional scour, they are teristic mineral staining (most commonly by iron the most common criterion for recognizing ancient minerals) at or just below their surface, which can hardgrounds.’18 draw attention to the surface in outcrop, this is not Many hardground-related features, such as mineral always present (particularly in coarser-grained staining, can occur in non-hardgrounds.19 It is for this and lithologies). In the absence of such staining it is the other above-quoted reasons that we have emphasized the presence of the boring and encrusting fauna borings in our search for hardgrounds. Apart from being alone that testifies at the macroscopic level to the much more conspicuous than encrusting fossils, these TJ 18(3) 2004 83 Countering the critics Field study of purported hardgrounds — Woodmorappe and Whitmore Field study of purported hardgrounds — Woodmorappe and Whitmore Figure 8. Closeup of the boreholes in the bryozoan-encrusted brachiopod and the bryozoan from figure 7. The minute borings are Figure 6. Bored corals situated next to other benthic organisms that outlined by circles. Quarter for scale. had escaped boring. Penny for scale. Implications of hardground borings The ichnotaxa of Paleozoic macroboring traces is acknowledged to be confused and subjective.24 The 0.5– 3.0 mm diameter boreholes prevalent in Lower Paleozoic fossils and hardgrounds have traditionally been assigned to the ichnogenus Trypanites (see illustrative photo online).25 Bromley26 has proposed a very broad definition of this ichnotaxon, one that encompasses most of the Phanerozoic and subsumes a considerable variety of unbranched, quasi- cylindrical cross-sectional shapes of the stated range of diameters. In contrast, Palmer et al.27 suggest that Paleozoic Trypanites should properly be transferred to the ichnogenus Figure 7. Small slab containing bored coral Grewingkia (top left) Palaeosabella, as it differs from Mesozoic Trypanites in and a bored brachiopod (far right bottom), in front and in focus. In shape and length. Unfortunately, Palaeosabella itself has a the background can be seen a bored bryozoan-encrusted brachiopod history as a confused taxon.28 To complicate matters further, (far left bottom, circled) and a bored bryozoan (middle left, circled). another boring ichnogenus found in the Cincinnatian, The superimposed white circles each have an inner diameter of 6 Sanctum, has recently been described.29 To avoid getting mm. Quarter for scale. involved in taxonomic arguments, our usage of Trypanites hardground trace fossils are known to be much more in this report is in accordance with the broad definition numerous than individual encrusters. In fact, an areally- advocated by Bromley. normalized census of Cincinnatian hardgrounds shows While searching for borings in the carbonate lithologies, the borings to be several times more common than all we were careful not to mistake vague depressions and soft- hardground body fossils combined.20 Furthermore, some sediment burrows for borings. All three can be common on lower Paleozoic hardground horizons lack