Peyote, La Popa, and a Marine Graveyard
Friendly fossils
surprised us on our scavenger hunt around La Popa basin (La Popa, Nuevo León,
México). Bivalve cross sections:
We made friends with
some caballos (not to be confused with cebollas), chivos y vacas (and some
minor trespassing, as an enthusiastic geologist or caballo whisperer may do):
And then we found
ourselves in the Coahuila fold belt (7). See those wild anticlines? So wild in
fact we may call them nappes. Imagine dancing wildly, so wildly
indeed, that your shag carpet bunches up in a groovy, overturned manner.
Now imagine a
tectonic-boogie: across the continental plates, across the room, the Gulf of
Mexico opens up (Triassic-Jurassic time, ~180-130 Ma) and with its force, its
wildly dancing 'foot', it pushes and bunches up the crust hundreds miles to the
west to form: the Sierra Madre Orientales.
Today these
carpet-ruffles are clear in the near-vertical tilted evaporite-nucleated anticlines
of Potrero Chico, Los Muertos or Venado.
The red circle on map below - the area of our fossil find. This spot isn't hard to find on google maps- that El Gordo diapir sticks out like a... salt dome! (Area GPS: 26.008135, -100.814120).
External mold fossils: one current gastropod, three gastropods and one bivalve from the Muertos or Potrilleros formation.
Baculites ovatus of the lower Maastrichian Potrerillos
formation. I think. (See the following stratigraphic column. The groovy
ceratitic suture patterns on this particular ammonite imply late Permian to
Triassic ammonoid (see next photo), though I believe these fossils lie in the
Cretaceous Potrerillos formation.
Can you envision a little
seaway teeming with foraging ammonite right here? With the inspiration of local
peyote and a geology guide, anyone could. (Or perhaps virtual reality goggles
if you've got a drug screen coming up).
Suture patterns by epoch
(3).
Angled cross section of
a gastropod, probably. Looks like a pinecone.
Below, a generalized
stratigraphy of the La Popa basin (2). It's important to explain the
significance of strat columns to aspiring geologists - much can be lost in
translation when I throw about terms and graphs without explaining what certain
concepts.
For example - I overhear two folks
chatting about the quarter back Johnny Stone of the Steelers overtaking the
defense Jad Bumble of the Astros, I really won't know at all what you're
talking about. My football exposure is limited to brief glimpses, distracted
stares at the local watering hole. I know enough to know one of those those
teams isn't even a football team.
I am not fluent in
football...
...just like some don't
speak geology, but it's only because they haven't trudged about landscapes
pondering the rites of passage all the rocks of the world go through, examining
talus slopes and researching your curious finds later for hours on Research
Gate!
The landscape begins to
speak to you - much like a football game makes sense if you've grown up in a
football friendly home.
At the surface, a strat
column looks like an ordinary piece of paper with colorful figures and shapes.
Nothing too interesting.
Your imagination is key
here. What does the Potrerillos formation contain?
Baculites ovatus.
I've enjoyed imagining living during the Maastrichian while standing on this terrestrial basin in 2018 - it once was a shallow sea. I envision how quiet it was 70 million years ago. No jumbo
jets, zooming Mazdas, rustling box trucks driving avocados North. (Although, a
world without avocados is not something that 'neat' to envision.)
I think of swimming in
this ocean and diving for Baculites ovatus, and of the the
vast shallow sea lapping onto shores of the mature (230 million years old at
this point) Sierra Madre Orientales that crumpled up during the Pennsylvanian (note: fact check this).
This map is a bit early
for us - a late Cretaceous (105 Ma) paleographic map, but re the Potrerillos
formation is Maastrichian (70 Ma, early Cretaceous).
Still, the shallow
seaway covers our study area of Nuevo León, and you can see why today we find baculites
fossils scattered about the area.
Find us on this
Cretaceous (145-66 Ma) map:
A strat column evokes
these maps and mental images in a geologist's mind. It is more than plain
run-of-the-mill writing.
Our gastropods, ammonite
and bivalve precursors were likely floating about this Cretaceous (late
Maastrichian, 70 Ma) seaway in the red circle:
A closer Maastrichian
map, our approximate area of exploration in red circle. Ignore the red
"Study Area" for now - that's for another study. Modified after
Goldhammer, 1999 (6)
What
a wonderful photograph! I would have loved taking part in the gravity data
collection (8)
With folds we find basins.
Avery and I walked perhaps two kilometers down a gravel road... the towering La
Popa weld so close yet so far away, looming in the distance:
La Popa basin has fallen
under inspection by a few field camps: in 2007, University of Alabama students
ventured to this basin to "examine superbly exposed structures and Lower
Cretaceous to Lower Tertiary strata that record mobilization of Upper Jurassic
Minas Viejas Formation evaporites. Diapiric evaporite structures in this basin
are among the best-exposed analogs to subsurface hydrocarbon traps in the Gulf
of Mexico and elsewhere."
As for the peyote...
evidently it grows in this area, but we were distracted by those gastropods. I am saddened by the K-T boundary and ammonite extinction... but happy to stumble upon my favorite cephalopods in the field!
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References For Us All
Geological map of La Popa Basin,
https://www.researchgate.net/figure/Geological-map-of-La-Popa-Basin-Study-area-Fig-4-outlined-in-blue-La-Popa-salt-weld_fig1_258646064
- Generalized stratigraphy of La Popa Basin. https://www.researchgate.net/figure/Generalized-stratigraphy-of-La-Popa-Basin-modified-from-Lawton-et-al-2001_fig8_258646064
- Harold H. Levin, Feb 2006, Life of the Mesozoic http://higheredbcs.wiley.com/legacy/college/levin/0471697435/chap_tut/chaps/chapter14-04.html
- Paleogene Period, http://www.geologypage.com/2014/04/cretaceous-period.html
- https://plos.figshare.com/articles/_Paleogeographic_maps_of_North_America_during_the_A_late_Campanian_8764_75_Ma_and_B_late_Maastrichtian_8764_65_Ma_/267596
- https://www.researchgate.net/figure/Paleogeographic-map-Maastrichtian-paleogeography-of-northern-Mexico-and-southern-Texas_fig3_242399122
- https://www.researchgate.net/figure/Geographic-and-tectonic-map-of-NE-Mexico-with-major-structural-features-and-sedimentary_fig7_258646064
- Geophysical Modeling of the Surroundings of La Popa Basin, NE Mexico, with Gravity and Magnetic Data. Vsevolod Yutsis, Antonio Tamez-Ponce and Konstantin Krivosheya, 2011.
- Stratigraphy and Origin of the La Popa Basin, Nuevo Leon and Coahuila, Mexico. Timothy F. Lawton. http://archives.datapages.com/data/specpubs/memoir75/m75ch09/m75ch09.htm
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