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Yorktown Formation, Zone 2, Rushmere Member


MikeR

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Approximately 4.5 million years ago, the Albemarle basin in North Carolina was a cool temperate ocean much like that off the coast of Alaska today. Although cold, the offshore waters supported a rich and diverse ecosystem due to the upwelling of deep nutrient rich water. Placopecten clintonius scallops along with young Chesapecten jeffersonius clapped their shells together in an attempt to escape from shell crushing rays. Chesapecten as they aged would eventually develop thick heavy shells and settle onto the ocean bottom to live the life of a sessile filter feeder forming dense beds along with the giant oyster Ostrea compressiostrea which lived a similar life. The predatory snail Ecphora quadricostata moved through the soft mud in search of bivalve prey. It would use its foot to envelope clams or slow snails and then using a rasping radula, drill a hole into mollusk shells giving it access to its victim’s vulnerable internal organs. Large whales swam these waters and were preyed upon by giant megalodon sharks and seals played in the water while searching for food. Bones and teeth of predator and prey scattered the sea bottom.

Global climate conditions however would change this idyllic setting. Temperatures began to drop as did sea level when polar ice caps and glaciers began to increase. What were near-shore seas became land and stayed that way for 500,000 years. When temperatures began to increase and the Albemarle basin became ocean once more, the whales were gone and megalodon extinct. Ecphora quadricostata survived but not Placopecten. Chesapecten and Ostrea returned but changed by evolution due to the environment that they retreated to during this ice age. The water was still cold but warmer. A richer molluscan fauna now existed but with much fewer sharks and fish as the waters were possibly less productive. A physical barrier separated subtropical waters in South Carolina from temperate conditions in Virginia however warm cycles would occur allowing for exotic southern species to thrive in the north at least for awhile. Barrier islands formed changing the conditions in this area of North Carolina from open marine to brackish. The molluscan fauna was replaced by species adapted to lower salinity, but returned when global temperatures raised sea levels once more. The late Pliocene however, was a tumultuous time. The final closure of the Isthmus of Panama was occurring changing oceanic currents forever. Whether this changing ocean pattern increased global ice or some other factor was the cause, the resulting sea drop was rapid and extreme with as much as 80% of the molluscan fauna becoming extinct. When the seas returned once more towards the end of the Pliocene, only a handful of species were present and great lineages such as Chesapecten and Ecphora were forever gone.

One of the reasons I enjoy fossil collecting is the story that rocks and fossils tell. The scene that I just described is depicted within the Yorktown Formation found within northeast North Carolina and Virginia. The first paragraph describes the topic of my last post, Zone 1 Yorktown Formation, the second paragraph and which I will be reporting in this entry, Zone 2. W.C. Mansfield divided the Yorktown into two biozones (Zones 1 & 2) while Ward and Blackwelder further divided Zone 2 into three members the Rushmere, the Mogarts Beach, and the Moore House. The Rushmere member outcrops along the Roanoke and Meherrin Rivers which I have been writing about in my last few posts. The Mogarts Beach member is present in one spot on the Meherrin, but since it is the brackish silty clay that succeeds the Rushmere, only the bivalve Mulinia congesta (Conrad, 1833) occurs in any great numbers and won’t be discussed here. The Moore House member represents the return of marine conditions and the Zone 2 fauna to the area that I also mentioned above. It includes the fantastic shell beds formerly exposed in the Chuckatuck quarry in Virginia but since it does not outcrop along the Meherrin or Roanoke Rivers, I will report about it at a later date.

I have collected the Rushmere member of the Yorktown Formation across North Carolina and Virginia. Zone 2 guide fossils include Chesapecten madisonius (Say, 1824), Chesapecten septenarius (Say, 1822) Ostrea raveneliana Tuomey & Holmes, 1855, and Ecphora quadricostata (Say, 1824) (fig. 1). Along the Meherrin River the contact between Zones 1 and 2 can be readily observed. The Rushmere exposures here are typical of this member; silty sands thick with shells of large bivalves and a few gastropod species scattered here and there (fig. 2). Of note is the presence of Glossus fraternus (Say, 1824), a taxa which is today represented by only a single species found in the North Atlantic and indicating that perhaps the Rushmere here at the Meherrin was colder than coastal waters today.

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Figure 1. Guide fossils of the Rushmere Member of the Yorktown Formation. A) Chesapecten madisonius (Say, 1824), loc. 1015, Hertford County, North Carolina; B ) Chesapecten septenarius (Say, 1822), loc. 1011, Martin County, North Carolina; C) Ecphora quadricostata (Say, 1824) [image courtesy of nala] ;) , loc. 1011, Martin County, North Carolina.

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Figure 2. Fossil gastropods from Locality 1015, Rushmere Member of the Yorktown Formation, Hertford County, North Carolina. A) Boreotrophon tectricus (Conrad, 1832); B ) Scalaspira strumosa (Conrad, 1830); C) Ptychosalpinx altile (Conrad, 1832); D) Urosalpinx trossula (Conrad, 1832); E) Lyrosoma sulcosa (Conrad, 1830).

The Rushmere is also present on the Roanoke River however the fauna is somewhat different than most that I have collected and therefore quite interesting. At locality 1010, the Rushmere sits directly upon Upper Cretaceous sediments probably Black Creek Formation. Although macrofossils are not present in the Cretaceous here, vertebrates are plentiful along the lower Rushmere contact. Since the Zone 2 Yorktown is vertebrate scarce, bones and teeth were probably derived from Zone 1 Yorktown or the Eastover Formation and deposited as lag during the beginning of the Rushmere transgression. Some in my party found small meg teeth at this site. I found a mid-sized but very worn meg, bone pieces, and a nice dolphin tooth (fig. 3) that I identified based upon an almost identical tooth figured in the VMNH Lee Creek publication (Whitmore & Kaltenbach, p.22, fig. 66a).

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Figure 3. Cf. Turiops sp. Tooth. Mio-Pliocene lag at base of the Rushmere Member of the Yorktown Formation. Loc. 1010. Halifax County, North Carolina.

I found several unusual shells at Roanoke River locality 1011 (fig. 4). Larger gastropods were common particularly carnivorous volutes and moon shells. Although I have collected these in the uppermost Zone 2 at Chuckatuck and they have been reported in the Rushmere, I have never personally collected volutes in the Rushmere. Preservation was not the best, but they were unusual enough to collect (fig. 5). An unusual Chesapecten is also found here. The population of Chesapecten madisonius at this locality has a maximum of 12 ribs compared to the usual 16 (fig. 6). The difference can be seen by comparing figure 6 to figure 1A. Lauck Ward who originally named Chesapecten has found these only in this area and thinks they are a hybrid ("race") of C. madisonius and C. septenarius, probably deserving a subspecific status (L. Ward, personal communication). I find that they are very similar to Florida Chesapecten and I will describe these in more detail in the future, but the presence of a more diverse gastropod population along with Chesapecten resembling southern forms could indicate a period of intermittent warmth. This is only conjecture on my part but if so, these conditions were not to last as the regression and then transgression 800,000 years later that is captured in the Chowan River Formation was to decimate this fauna.

blogentry-1906-0-36914600-1308164062.jpgFigure 4. Shell bed at Locality 1011, Rushmere Member of the Yorktown Formation, Martin County, North Carolina.

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Figure 5. Fossil gastropods from Locality 1011, Rushmere Member of the Yorktown Formation, Martin County, North Carolina. A) Volutifusus obtusa (Emmons, 1858), B ) Volutifusus mutabilis (Conrad, 1834), C) Euspira perspectiva (Rogers & Rogers, 1837).

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Figure 6. Chesapecten madisonius subsp.? Ward pers. comm. Loc. 1011, Rushmere Member of the Yorktown Formation, Martin County, North Carolina.

Molluscan fauna lists from the region below:

Mollusca Locality 1011.pdf

Mollusca Locality 770.pdf

Mollusca Locality 1013.pdf

Mollusca Locality 1015.pdf

REFERENCES

Campbell, Lyle. 1993. Pliocene Molluscs from the Yorktown and Chowan River Formations in Virginia. Virginia Division of Mineral Resources Publication 127.

Christie, Max. 2009. Ecological Interactions Across a Plio-Pleistocene Interval of Faunal Turnover: Naticid Cannibalism North and South of Cape Hatteras, North Carolina. Departmental Honors in Interdisciplinary Studies Thesis, The College of William and Mary.

Mansfield, W.C., 1943 [1944]. Stratigraphy of the Miocene of Virginia and the Miocene and Pliocene of North Carolina in Gardner, Julia ed. Mollusca from the Miocene and Lower Pliocene of Virginia and North Carolina. USGS Professional Paper 199A, p. 1-19.

Molnar, Peter. 2008. Closing of the Central American Seaway and the Ice Age:

A critical review. Paleoceanography Volume 21.

Petuch, Edward J. 2004. Cenozoic Seas. CRC Press.

Schmidt, D. N., 2007. The closure history of the Panama Isthmus: Evidence from isotopes and fossils to models and molecules. In: Williams, M., Haywood, A. M., Gregory, J. F., and Schmidt, D. N. Eds.), Deep time perspectives on climate change - marrying the signal from computer models and biological proxies. Geological Society of London, London.

Ward, Lauck W. 1992. Tertiary Molluscan Assemblages from the Salisbury Embayment of Virginia. Virginina Journal of Science, Volume 43, no. 1B.

Ward, L.W. and Blackwelder, B.W. 1975. Chesapecten, a New “Genus of Pectinidae (Mollusca: Bivalvia) from the Miocene and Pliocene of Eastern North America. USGS Professional Paper 861.

Ward, L.W. and Blackwelder, B.W. 1980. Stratigraphic Revision of Upper Miocene and Lower Pliocene Beds of the Chesapeake Group-Middle Atlantic Coastal Plain. USGS Bulletin 1482-D.

Whitmore, Frank C. Jr and Kaltenbach, James A. 2008. Neogene Cetacea of the Lee Creek Phosphate Mine, North Carolina in The Geology and Paleontology of the Lee Creek Mine, North Carolina, IV. Virgina Museum of Natural History Special Publication No. 14.

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Mike, this is an outstanding article! It's full of information relevant to so many that want to understand more about the geology in that area. Thanks for posting it.

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Great article, very well done! Your descriptions of the paleoenvironment are very vivid! Were the waters of the zone 1 deposition colder because there was no Gulf Stream yet since the Panama Isthmus was not yet closed?

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Your report is very good. It looks like you’ve put a lot of work into this one and the others. I’ve been to two outcrops on the Roanoke (I stumbled upon them by accident while boating) and the thing I really wished at the time was that I knew more about mollusks so I could identify the units that were present. Recently I’ve been visiting some outcrops on the Tar River and some of the nearby streams and have been trying to collect the pectens to identify so I can narrow down the identities of the units. I’m slowly learning and am enjoying it. I’ve been using Lee Creek volume 2 for the identifications of the pectens.

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Great article, very well done! Your descriptions of the paleoenvironment are very vivid! Were the waters of the zone 1 deposition colder because there was no Gulf Stream yet since the Panama Isthmus was not yet closed?

Perhaps. There are two articles in my references (one by Molnar and the other by Schmidt) that discuss the closure. A lot of evidence is used to predict when this occurred i.e. fossils, volcanic ash, etc but the exact time is not known. Both report that the connection started sometime in the Miocene and was complete by evidence of mega-mammal exchange in the Late Pleistocene, but there were intermittent periods of closure and open water between those two times. I think that these constant events were influential with ice ages and wreaked havoc on living organisms, but I tend to write in generalities regarding this as I am an interested enthusiast as opposed to an expert.

Mike

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Your report is very good. It looks like you’ve put a lot of work into this one and the others. I’ve been to two outcrops on the Roanoke (I stumbled upon them by accident while boating) and the thing I really wished at the time was that I knew more about mollusks so I could identify the units that were present. Recently I’ve been visiting some outcrops on the Tar River and some of the nearby streams and have been trying to collect the pectens to identify so I can narrow down the identities of the units. I’m slowly learning and am enjoying it. I’ve been using Lee Creek volume 2 for the identifications of the pectens.

I have wanted to collect on the Tar River but have yet to have the opportunity. Although it is out of print, the best reference to identify Yorktown mollusks is Campbell's. I have contemplated posting a "collectors guide to Chesapecten" but I want to write about Florida Chesapecten first.

Mike

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A lively and colorful description of life on earth approximately 4.5 million years ago :o Very nice report Mike!

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great work, very engaging, thanks for a spirited read, Im only sorry I hadnt noticed your writings sooner, Carmine.:)

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That was a fantastic bit of work on your behalf and I really enjoyed reading it. The pictures as well as the information provided are exceptional to say the least. Thanks a lot.

Bobby

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