An educational (geology/paleontology) trip report on a SC Pliocene land site

I’ve been studying the geology and fossils from a South Carolina land site that ditchweezil has referred to as the Pliocene land site for the past year.  There are at least two undetermined siliciclastic fossiliferous marine deposits, which I will informally refer to as the Parachucla Shale and Teleost Sands. There were several other overlying deposits containing undetermined sandy beds with no visible fossils with the exception of one muddy bed (Diatomaceous Shale) that contained microscopic diatom fossils. The oldest marine deposit found at the base of this site is the Parachucla Shale and it is likely of Late Oligocene/Early Miocene in age. The lithology of the Parachucla Shale from this site can be described as a mudstone with the following description: dark grey, dark green, firm-hard, blocky, earthy, non-calcareous, and fossiliferous.  Fossil burrows are abundant in the subsurface of the Parachucla Shale as well.  The age of the Parachucla Shale was restricted to no younger than Late Oligocene/Early Miocene based on an overlying unconformity referred to as Unconformity Z, but it is possible that the Parachucla Shale is older than Late Oligocene.

Unconformity Z comprises a lag deposit containing pebble to cobble sized phosphate clasts, discoidal and well-rounded quartz pebbles and cobbles (flat potato shaped quartz rocks), and reworked marine fossils (worn bones). The discoidal pebbles are fluvial derived (river channels) from a formation known as the Upland Unit, which is located in the Piedmont region of South Carolina.  The Appalachian Mountains experienced a literal face lift due to tectonics during the Late Oligocene/Early Miocene and were said to be equivalent to the heights of Mount Everest. When sea levels dropped during the Late Oligocene/Early Miocene the discoidal pebbles from the Upland Unit were fluvially deposited across the Coastal Plain of South Carolina and deposition may have likely continued up until the end of the Miocene. Discoidal pebbles are also evident in the upper facies of the Chandler Bridge Formation (Late Oligocene) of South Carolina. The reworked fossils in the lag deposit consist of mainly gastropod molds, whale, dugong, fish, and shark remains. It is hard to confirm an actual age of the lag deposit due to the broad range of shark species that can be found in both the Paleogene and Neogene. In my opinion, the lag deposit at the Cottageville Site contains reworked fossils of Late Oligocene – Late Miocene based on the faunas found in the lag deposit. I say Late Oligocene due to the reworked Carcharocles teeth with denticles (cusps) near the basal edges of the crown, which may represent either Carcharocles chubutensis or Carcharocles angustidens. And a Late Miocene age was suggested because reworked Galeocerdo cuvier and Carcharodon carcharias teeth are present in Unconformity Z. The first occurrence of Galeocerdo cuvier is not well documented in the literature, but they have been reported from Late Miocene deposits. The importance of reworked Carcharodon carcharias in Unconformity Z will be emphasized as you read further down in the report. Phosphates found in Unconformity Z were formed when sea waters cooled during the Miocene. The final phase of Unconformity Z was completed with a transgressive sea that occurred in the Early Pliocene – Middle Pliocene, which undercut underlying stratigraphic units containing fossils of Late Oligocene – Late Miocene age, and mixed them into a lag deposit, forming Unconformity Z.  The Teleost Sands were likely deposited during the Early - Middle Pliocene. The lithology of the Teleost Sands can be described as unconsolidated sand with the following description: dark greenish-gray to dusky-green, fine-grained, grains moderately well rounded, well-sorted, and fossiliferous.

Shark teeth and bony fish remains are the most abundant fossils from this site. The Teleost Sands contains abundant fish remains, shark teeth (sporadic), whale bones (common) & teeth (rare), bird bones (rare), in-articulate brachiopods (whole specimens are uncommon), and sea turtle remains (uncommon) that exhibited exceptional preservation, which is rarely reported from upper Neogene deposits in this region of South Carolina due to the emerging-submerging sea cycles that have reworked and eroded away most of the upper Neogene deposits. While on the other hand, the Parachucla Shale does not contain abundant fossils (shark and fish remains are uncommon). The presence of Carcharodon carcharias in the Teleost Sands constrains the age of the Teleost Sands to no older than Early Pliocene, which is based on the theory that Isurus hastalis evolved into a transitional species known as Carcharodon sp. before evolving into Carcharodon carcharias. Carcharodon sp. shares both Isurus hastalis and Carcharodon carcharias morphological features and are restricted to the Late Miocene.  Unfortunately, the Neogene deposits in this region of South Carolina are poorly documented in the literature and further paleontological and geological research is needed.  I believe that the depositional period of the Teleost Sands closely matches the Duplin Formation (Early – Middle Pliocene) and represents a deep water facies (change in depositional environment) because the Teleost Sands does not contain an abundance of invertebrates (bivalves & gastropods), which the Duplin Formation is known for. The Teleost Sands and Parachucla Shale will need to be further investigated for microfossils in order to further constrain the ages of these deposits.