Transcript File
Senior University Geology Field Trip November 5, 2012 8:45 am – 4:30 pm Georgetown’s Amazing Geology with Don, Rocky, & Sandi Explore, Photograph, & Sample . . . . . world famous rocks modern perched water table modern aquifers supplying our drinking water 10, 000 years old black waxy glacial clays 135 million year old Rudist reefs 45 million year old Balcones Fault Our Travel Schedule 8:45 to 9:00 am 9:10 to 9: 30 9: 40 to 10:00 10:15 to 10:30 10:35 to 10:40 10:50 to 11:15 11:30 to 12:15 12:30 to 1:30 2:00 to 4:00 4:30 Briefing at The Oaks STOP 1 - Perched water table Berry Creek Pond STOP 2 - Black Waxy clays Walgreens, Williams Dr STOP 3 - Rudist reefs Lake Georgetown spillway STOP 4 - Reef in D.B. Woods road cut STOP 5 - Rudists and grasses Wolf Ranch Mall STOP 6 - Buda limestone cliff & faults Celebration Church LUNCH – French Quarter Grill STOP 7 - nner Space Cavern Return to Sun City Definitions Mineral: natural occurring constituent of all rocks Limestone: sedimentary rock composed of the mineral Calcite Shale: sedimentary rock composed of one or more Clay minerals Sandstone: sedimentary rock composed of the mineral quartz Ground water: rain water filling rock voids Water table: top of the ground water Aquifer: rock containing fresh water that can be recovered in a well Normal fault: fracture in bed rock with measurable displacement Stromatolites: Shallow water blue-green algae deposits Briefing at The Oaks, Sun City Age of the Rocks that we saw Cretaceous Period Eagleford Shale 120 feet Buda Reef 60 feet Del Rio Shale 60 feet Georgetown Limestone 100 feet Upper Dense limestone 30 feet Rocks at the surface in Sun City Edwards Limestone 380 feet Reef & Reef debris aquifer limestone 100 feet Comanche Peak limestone 50 feet Upper Walnut limestone 100 feet Lower Walnut limestone 100 feet Glen Rose Limestone 400 feet Trinity sandstone 0 to 100feet Ouachita Mountain complex folds and thrust faults Trinity sandstone aquifer 0 to 100 feet Geography when our limestones were formed about 115 million years ago Rocks Exposed at Sun City Berry Creek not eroded this deep Geological Cross Section Williamson County, Texas Springs Fossils you may have stumbled over Bryozoa (sea moss) Stromatolites (Algae) Brachiopods (symmetrical shells) Gastropods (snails) Pelecypods (clams) (asymmetrical shells) Cephalopods & Ammonites Corals On the road . . . Stop 1 - Perched Water Table at Berry Creek Pond Stop 1 - Perched Water Table Pavilion at Berry Creek Pond Berry Creek Pond (stock tank) Williams Dr. Gate Village Center Reef Reef water filled=a perched water table Stop 2: Holding pond at Walgreens on Williams Dr . . .the base of the pond is on impervious limestone! Stop 2: Waxy Clay on weathered Georgetown Limestone Walgreens on Williams Dr What’s the black surface dirt? Stop 2: Waxy Clay on weathered Georgetown Limestone Walgreens on Williams Dr black waxy clay weathered Georgetown limestone un-weathered Georgetown limestone Stop 2: Waxy Clay on weathered Georgetown Limestone 10,000 year old black waxy clays weathered & eroded from Rocky Mountains in New Mexico Stop 3: layered limestone with rudist reefs What rocks are these? Lake Georgetown Spillway Stop 3: layered limestone with rudist reefs Lake Georgetown Spillway Lake Georgetown Spillway North Rock Wall Stromatolites cap layer Stromatolite layer Eoradiolites Rudist Debris Bank Rudist Reef-forming Sea Shells Lake Georgetown Rudists Rudists: the exotic mound-building Rudist Clams that became extinct at the end of the Cretaceous Length: 6 inches to 3 feet! WHAT IS A RUDIST REEF? A clam-like colonial (successive generations build on top of each other giving the reef vertical strength that minimizes compaction of the reef when buried). The rudist sea shell is shaped and sized like an inverted old fashioned glass Coke bottle. They occur in the Cretaceous Period 145 to 65 million years ago and are the equivalent of corals of today and other geological Periods. The rudist fossil is the “limestone” shell of the marine organism and it is very delicate so it often weathersout leaving a round hole. Stromatolites: blue green algae Stromatolites grow in shallow water Modern Stromatolites Stromatolites preserved as rocks Stop 3: layered limestone with rudist reefs Stop 3: layered limestone with rudist reefs Stop 3: layered limestone with rudist reefs Lake Georgetown Spillway Stop 4: Limestone Reef Outcrop Road Cut, D.B. Woods Road D. B. Woods Road - reef outcrop . . . the reef ‘matrix’ resists compression from the rock layers above, maintaining its shape Stop 5: rudist fossils in country rock, Texas grasses Wolf Ranch Shaded Walk Stop 6: Eagle Ford gas source rock, Buda cliff, Texas Crushed Stone Quarry, Balcones Faults Celebration Church Overlook Balcones Faults Inner Space Cavern Celebration overlook Up Up Down Down Balcones Faults Inner Space Cavern Note: Black Waxy Celebration overlook Lunch: French Quarter Grill: Round Rock Stop 7: Inner Space Cavern Tour Stop 7: Inner Space Cavern Tour Balcones Fault Flint Dolomite Limestone ROCKS OF THE INNER SPACE CAVERNS - Limestone is the original rock type which when buried can have the Ca partially replaced by Mg due to the movement of fluids being expelled by compaction of the whole rock column creating dolomite. - Dolomite does not “fizz” in acid until you scratch it into a powder, then if “fizzes”. Otherwise, it look just like limestone. - Flint is silicon dioxide and is thought to be the silicon dioxide of sponge quills called spicules sort of like the quills of a porcupine, but a marine animal that shared the reef with the Rudists and other reef organisms. The thin, discontinuous flint layers are thought to be formed when a storm swept a number of individual sponges off the reef and deposited their dead bodies in a “quiet” place out of the way of the waves and tides. Since the spicules become plastic as the organic sponge bodies decay, the plastic layers of flint during burial and compaction are often squeezed into “lumpy” and discontinuous “blobs”.. The end of a perfect day!!! Self-drive Field Stop The next 4 slides describe a self-drive field stop that you could do with family or friends. The destination is Blue Hole City Park in Georgetown. On the drive from Wolf Ranch Shopping Center to the park, you will cross the Balcones Fault just east of Interstate 35. At Blue Hole Park, rudist fossils in the Edwards Limestone are visible on the rock outcrop (rock face). Enjoy!! Self-drive Field Stop: Blue Hole City Park Balcones Faults & Edwards Limestone Rudist Reef Blue Hole Park Austin Ave Wolf Ranch Self-drive Field Stop @ Blue Hole Park: Balcones Fault Blue Hole Park Up Down Austin Ave Self-drive Field Stop @ Blue Hole Park: Balcones Fault Blue Hole Park Up Down Self-drive Field Stop @ Park: Edwards Rudist Reef