Geology by paddleboard and other geological (and paleontological) snapshots from the tropics

This Friday Sarah and I are heading to Playa del Carmen with my family for spring break - since 2018 we've been going to the Caribbean/tropical Atlantic for spring break, since the tropics are so damn close to Charleston and it's pretty affordable* to get to. We've done the Bahamas, Turks and Caicos, and Grand Cayman (twice) and seen some neat stuff. This time, we get to combine the tropical Caribbean with the terrestrial biodiversity of the Yucatan peninsula and the cultural/archaeological heritage of the Mayan civilization - I'm 1/4 Mexican, and while my Spanish is admittedly rudimentary, I love the food, history, and culture and this will be my third time to Mexico since I first went to Baja for Shark Week in 2019 and Mexico City a few months later. The best part about going to Playa del Carmen is the fact that we won't be anywhere near Cancun during spring break and away from all of those college students I have to be around during the school year!

 Regardless, I've been meaning to share these photos for quite some time, and since I'm getting excited for our upcoming trip to Mexico, here are a number of snapshots with no particular theme other than showcasing some neat outcrops I paddleboarded by and some various tropical fossils from Turks and Caicos in 2020 and Grand Cayman in 2022. The 2020 photos are taken with a GoPro and aren't great, but the 2022 shots I believe are all with an Olympus TG6 and the photo quality is quite good.

*For our very, very delayed honeymoon (delayed because of my Ph.D.; we wanted to go to Rarotonga in the Cook Islands, just a few hours flight from NZ, but couldn't afford it; after moving to South Carolina it took a few years to scrape anything together owing to how expensive rent is) - we thought about going to the Bahamas, but was uneasy about plane tickets. One day while walking past my colleague's office door I saw a map of Florida and saw that Grand Bahama island is actually on the same map and only like 90 miles offshore - we could probably take a boat! There's a three hour ferry ride to Grand Bahama from Fort Lauderdale and it only costs like 100-150$  per person, round trip. Once there hotels cost ~90-200$ per night. We were able to do five nights and all travel for about 1200$.

A nice afternoon paddle and workout on the canals on the south side of Providenciales in Turks and Caicos - this is taken March 18, 2020 - we were supposed to be enjoying vacation in the tropics but lockdowns started happening across the USA after we left. We flew on a plane weeks before masks were even recommended or the virus thought to be spread through respiratory droplets, unlike SARS-CoV-1. Needless to say our 2020 trip was not the most relaxing. The day we arrived was the first day of panic-buying in grocery stores on the island.

 

 Shortly after venturing out down the canals I noticed these beautiful exposures of cross-bedded oolites - oolites are a type of carbonate sand (oolite) made out of ooids, which are tiny limestone grains that grow through time. Many beautiful white sand beaches in the Bahamas and Turks and Caicos are made of ooids.

It may be difficult to make out here, but this photo shows that the lower part of the section has smaller scale cross-bedding than that above - trough cross-bedding, with individual beds terminating over a distance of 1-2 meters instead of being dominated by laterally extensive, seaward-dipping planar laminations above. Trough cross-bedding suggests subtidal migration of ooidal sand dunes to me, and the planar laminated oolite above is likely instead a beach deposit - the swash zone on the beach itself where the waves go up and down in the intertidal zone. If this is correct then the 3-4 meter sequence here represents shoaling from subtidal to relatively shallower intertidal deposits. I'm not certain whether or not these are early Holocene or Pleistocene sediments. There isn't much published on the sedimentology of this island, which I find surprising.

What a geologist on vacation sees! This is a large cross-bed in the upper unit, showing a bit more complex bedding than normal - there was likely some sort of channel here perhaps. Click on the lower photo to see my annotations and the pattern of bedding. According to some very, very coarse geological maps there are Pleistocene beach deposits along the south side of the island in this area, so perhaps the swash zone of the beach is a reasonable interpretation.

 A couple of queen conch shells (Lobatus/Strombus gigas) in ?Pleistocene beach deposits on the north side of Providenciales.

 And modern queen conch shells in modern ooidal sand on the south side of Providenciales. Here's a tip for those who want a nice conch shell to bring home (provided the country allows it - which Turks and Caicos does not) - find a seafood restaurant that serves conch fritters (an amazing appetizer in the Caribbean) and go exploring nearby. Frequently, you'll find some piles of shells (and trust me - you'll smell them first, so walk upwind - if you dare!) and will be able to pick out a couple of nice trophies.

A queen conch (Strombus/Lobatus gigas) shell from the Ironshore Formation near Smith's Barcadere in Grand Cayman. I have no idea if collecting such a specimen is legal or not so I left it.

Several queen conch shells in situ within the Ironshore Formation at Smith's Barcadere on Grand Cayman.

 

A large brain coral - perhaps Pseudodiploria (symmetrical brain coral), one of the more commonly seen large brain corals in the area today. However, there are certain corals that are completely extinct today in the Caribbean that occur in the Ironshore Formation.

 Many transported brain coral fragments mixed together with queen conch shells - under the Dunham classification scheme for limestones, since the large bioclasts are touching, I think this would be classified as a rudstone. None of these corals looked to be in situ.

 

A relatively large top snail - Lithopoma most likely, species uncertain owing to the age. It seems to be ornamented with larger, smoother ribs than extant Lithopoma americana. The modern snails below are beaded periwinkles, Cenchritis muricatus, which were perhaps 1 cm in length, and this fossil shell was about 2-3 cm in diameter.

 

 At the other end of the island we chanced upon another exposure of the Ironshore Formation at Barefoot Beach after our last round of snorkeling on our 2022 trip. I found one of the most interesting fossils of the trip - this fossil Pocillopora, a cauliflower coral! Pocillopora is perhaps one of the most characteristic corals found today in Indo-Pacific reefs - but preserved in the Caribbean during the Pleistocene. Fossils like this one indicate that it became regionally extinct sometime after 125,000 years, which is the age of the Ironshore Formation. Similar specimens dating to the Sangamonian interglacial ~125 Ka have also been reported from rocks of the Dry Tortugas at the southwest end of the Florida Keys.

 

 

An excellent but small brain coral in the Ironshore Formation, Barefoot Beach, Grand Cayman.

 

 

These last two are admittedly more geomorphological than geological, but I never get tired of these extreme wave cut overhangs (coastal notches) in tropical limestone. Note the solution pitting on the roof of the notch -this indicates that on some level dissolution of limestone is occurring. As it happens, this is actually a bit of a controversy in tropical geology: seawater in the tropics is saturated with respect to calcium carbonate and quite basic, so how is there any dissolution occurring, which requires a low pH? pH changes, perhaps dissolution occurs during the winter when the water is undersaturated - perhaps this dissolution is mediated by algae. Most of the notch formation is probably from sandblasting, but there is *some* degree of dissolution happening, and in other tropical settings estimated to account for 10% of notch formation (presumably by volume). I wondered how the hell these notches formed because the waves are not large, and the notch is very shallow and very deeply incised (laterally anyway). Along temperate coastlines notches tend to be much shallower, and without much solution pitting.

 

Updates from the CCNHM prep lab: mystery bone (Pliocene Purisima Formation, California)

Frequently I am able to identify vertebrate fossils in the field based on their cross-section or if a little bit of the bone is exposed: when dealing with fieldwork in the Miocene-Pliocene Purisima Formation in Northern California, perhaps 80-90% of vertebrate fossils are ribs and vertebrae from cetaceans (whales and dolphins). If I can tell something is a whale rib or vertebra, I move on - these bones are not particularly useful or informative. And, to be clear, I'm talking about whether or not such a specimen could be published - does it represent a new record for the fossil assemblage? Is it diagnostic enough to name a new species from? If I can tell it's something else - a flipper bone, particularly a humerus, radius, or ulna from a whale, dolphin, or pinniped - that's something I can work with. Some vertebrae are even useful - particularly the atlas or axis from a cetacean. Skull bones, earbones, partial skulls, and complete skulls are desirable (and in that order). Skeletons are most exciting - but very, very rare in the Purisima Formation owing to the rapid skeletonization and disarticulation that occurs in shallow well-oxygenated marine shelf settings. Usually, I can narrow down a marine mammal bone to one of these categories - 'not interesting', 'possibly/probably worth collecting', or 'holy shit!' A vertebra, possible skull bone, and a nice skull would fall into these categories (respectively). 

All of this is a long-winded way of saying when I find something in the middle category - probably interesting - I usually leave the field with the fossil, but maybe not a clear idea of what it is.That usually changes after a little bit of preparation. Rarely, do I ever continue to prepare something and have no clue what the hell it is after much of the bone is exposed - this has happened to me with Purisima Formation fossils only once before - a weird bone fragment I collected in 2007, which is still not identified. Well, it happened again! I am once again stumped. And I LOVE getting stumped. When I get stumped, it generally means that whatever I'm trying to identify is either 1) a tiny uninformative piece of something really big or 2) something really unusual and exciting. 

 

The mystery bone in situ in the Purisima Formation.

How I initially interpreted the specimen in the field - as a tuskless, isolated left maxilla of a small specimen of the walrus Valenictus (specimen at San Diego Natural History Museum shown upside down for comparison). I thought the part labeled in blue was the edge of a tusk socket and the foramen in red as the infraorbital foramen. However, there was quite a bit more, and the other side was somewhat cylindrical.
 

In December I collected a relatively large bone fragment from near the top of the Purisima Formation under a state parks permit - it's a locality I've collected and studied extensively, but is still providing new surprises and challenges. When I first spotted the bone, I thought that maybe - just maybe - it was the tusk socket from a partial walrus skull. When I pulled the whole thing out, it was certainly not that - the back side was smooth, finished bone, almost resembling a rib - though it was clearly not a rib. I shrugged and then wrapped it up in paper towel, and when I got back to my folks' house, wrapped it in a bit more newspaper and in an oversized ziplock bag for the flights back to the east coast. On Friday of last week I opened it up, and thought if I started prepping it I would have an ID before I went home for the weekend - but I was wrong.

 The mystery bone in three different views, as I left it Friday night.

  The mystery bone after a little bit more preparation earlier this week.

 

So far, there are a few possibilities - but let's discuss what it is NOT first: it's pretty clearly not a vertebra fragment or a rib, unless it's a very messed up (e.g. pathologic) rib. I also do not think it's part of a baleen whale mandible, though there are some vague similarities. I've also compared it with limb bones from the sea cow Hydrodamalis and walruses, for which it is no match, and can confirm immediately that it's not a part of a cetacean forelimb bone. So, some possibilities in no particular order:

1) Partial walrus pelvis. Several candidate walruses are present in the California Pliocene, and they have big, chunky pelves. This would be neat, but it's not quite a good match - there isn't anything that looks like an acetabulum (hip socket).

2) Unusual baleen whale mandible, posterior end. The expanded feature at one end looks like it could be a broken mandibular condyle, and there is a bony flange below that somewhat resembles an angular process - however, there is no mandibular canal.

 

3) Sea cow premaxilla. There are some similarities between this and the left premaxilla of Hydrodamalis, which is already known from the Pliocene of California - Hydrodamalis cuestae, a larger extinct relative of the recently extinct Hydrodamalis gigas. The match isn't perfect, and some parts bother me. One problem is that the texture seems to be somewhat too porous.

The squamosal of baleen whales is quite diagnostic, and there are some similarities with the squamosal of a dwarf right whale, but it is not a great match.

Comparison of mystery bone with the left squamosal of Balaenula astensis, a dwarf right whale from the Pliocene of Italy (photos by Felix Marx). The mystery bone is quite flattened from front to back. However, the small size of the mystery bone would necessitate this specimen being very small, likely a juvenile, and perhaps some of the differences in morphology could be explained by growth.

4) Squamosal from a very tiny dwarf right whale. In this scenario the expanded process I initially mistook for a tusk alveolus would be the glenoid fossa (jaw joint) of the squamosal, and the small flange opposite would be the zygomatic process. If this is accurate, it would be a very, very small right whale, smaller than the Pliocene taxon Balaenula, and perhaps as tiny as Balaenella brachyrhynus from the Pliocene of Belgium. However, there is one major problem here: the bone is just not wide or deep enough, and there's no hint of the occipital articulation - the occipital shield is the triangular part at the back of the skull, and there should be radially oriented ridges for the occipital bone along with a platform raised up from the rest of the squamosal.

5) And here we're starting to get desperate: baleen whale pelvis. There's a slim resemblance, but admittedly it is less than fantastic.

6) Some unusual sort of land mammal bone. This part of the Purisima Formation is late Pliocene, so there are already some large ground sloths and a slew of elephants present in North America. However, nothing immediately comes to mind that looks similar.

7) One last possibility, and I need to humble myself here: perhaps this bone falls into the first category from further above in the post - a smallish piece of something really big, now too small to recognize immediately.

There is more preparation to tackle, so hopefully after some more cleaning the identity will be clarified!