Archive for July 9th, 2013

Return to the Ora Formation

July 9th, 2013

8_MudVolcano070913MITZPE RAMON, ISRAEL–The last location Wooster Geologists in Israel visited today was on the southern edge of the Makhtesh Ramon structure (N 30.58209°, E 34.89375°). Here are excellent exposures of the Ora Formation (Upper Cretaceous, Turonian). This curious feature was a challenge to the students to interpret. I also got it wrong in my explanation on the outcrop, so listen up Steph, Lizzie and Oscar! The students are standing in a portion of the outcrop that is mud with suspended blocks of limestone. This is a cross-section of a diapir, or body of sediment that has moved upwards through the rocks that cap it. This was caused by water-saturated sediment being compressed by the sediments above, forcing it upwards through cracks and crevices. What I got wrong was that the flat strata on top of the mud was present when the diapir formed. (I said it came later.) The mud never reached the surface to become a mud volcano. This is why the resistant beds below are bent downwards — the upward force of the mud flow was stopped by the capping rock, thus deflecting the edges of the units below. A complicated story — which is one of the many things that makes the Ora Formation interesting.

9_Oysters070913Also in the Ora Formation at this same site is a half-meter-thick unit composed entirely of oyster shells. Many of the oysters are encrusted with other oysters and, who knows, maybe bryozoans as well. (And no, Paul Taylor, I didn’t see any here yet!)

10_Hardground070913The Ora Formation also has a fabulous carbonate hardground, which was a cemented seafloor surface. We can tell this particular surface was hard rock on the Cretaceous seafloor because of all those little holes. These are the borings of bivalves known as Gastrochaenolites. They could only be made by grinding away at a cemented substrate.

Hardgrounds, oysters, odd diapirs … opportunities for future study! Israeli geologists have done fantastic work with this unit, so there are many collaborations possible here.

Adventures in the Triassic: Exploring the Gevanim Valley in Makhtesh Ramon, southern Israel

July 9th, 2013

2_NordmarkiteStock070913MITZPE RAMON, ISRAEL–The second visit of the day for Wooster’s Team Israel 2013 was to the Gevanim Valley on the south side of the Makhtesh Ramon structure. This is a fascinating place where Cretaceous intrusions formed an uplifted dome exposing Triassic sedimentary rocks. It is a rare place to see abundant Triassic marine fossils. Our first stop was a nordmarkite stock intruded into the Gevanim Formation (Middle Triassic, Anisian). We always dedicate this image to our own Dr. Meagen Pollock who knows what nordmarkite is without having to google it.

3_GevanimRamonalinidSite070913Our first task was to locate the Gevanim Formation and examine the many specimens of the large bivalve Ramonalina ramanensis to look for rare Triassic encrusters. Above is an outcrop of the part of the Gevanim which has large numbers of this dark-colored, shoe-shaped clam.

4_Ramonalina070913Here are two nearly complete specimens of Ramonalina ramanensis. Alas, we found not a single encruster. The rumor that there are microconchids on these shells seems to be false. Science marches on.

5_LizzieSaharonim070913Above the Gevanim Formation is the Saharonim Formation (Middle Triassic, Anisian-Ladinian). Lizzie Reinthal is here standing near the base of it exposed in the western part of the Gevanim Valley. This is a very fossiliferous limestone and marl that is extremely well exposed here.

6_CephalopodsSaharonim070913Nautiloids and ammonoids are very abundant in the Saharonim. In fact, just about every large object in this exposure of the unit is one or the other. The coin in the image above is sitting on an ammonoid (a ceratite). The other fossils are internal molds of nautiloids.

7_SaharonimBrachiopods070913Our goal today, though, was to find terebratulid bachiopods with original calcite still preserved. We found dozens, a few of which are shown above. These are mostly of the genus Coenothyris. These specimens are destined for isotopic analysis in the laboratory of Dr. Pedro Marenco at Bryn Mawr College. Mission accomplished.

“The Carpentry” in Makhtesh Ramon: Unexpected columnar jointing

July 9th, 2013

1_JointedSandstone070913MITZPE RAMON, ISRAEL–At first glance this rocky outcrop in the middle of Makhtesh Ramon appears to be a typical columnar-jointed basalt. We’ve seen this many times on our blog (for example here and here). However, these rocks are entirely a quartzose sandstone. They have the typical polygonal joints of a cooled lava flow, but the rock is an unmetamorphosed sedimentary unit. This remarkable site is known as “the carpentry” (Haminsara) in the park.

How did these joints form? It is not from the sandstone melting and then cooling, like you’ll see in some places on Wikipedia. (And some people think this is basalt, which is a good reason for more interpretive signs in this place.) Likely it was a hydrothermal process by which superheated water from nearby intrusions warmed up the sandstone until it expanded a bit, and then it cracked along these joints during cooling. The sandstone was never heated to temperatures that would turn it into quartzite, much less liquid. Columnar-jointed sandstone is rare but not unique, as you can see here and here.

This was the first stop for the Wooster Geologists in Israel today as we explored parts of Makhtesh Ramon to follow up on various small projects.