Story In The Rocks A Conceptual (Rough and Greatly Simplified) Interpretation of the Geologic History of Heil Valley Ranch Open Space Setting the scene: Prior to the onset of the Laramide Orogeny, the Ancestral Rockies to the west (pushed up about 350 mya in response to the collision of continents forming the supercontinent of Pangea) had been eroded down nearly flat. The Cretaceous Seaway had totally inundated the entire area which is now Colorado, depositing thick marine deposits. About 12,000 feet of sedimentary material now blanketed the ancient metamorphic and igneous basement rocks. As the sea retreated to the east, the Fox Hills Sandstone and the Laramie Formation were the first terrestrial deposits of the newly emerging land pushed up by the Laramide Uplift. Laramie Fm.: estuary, marsh, and lagoonal Area of present central deposits transition to swampy coal forming Cretaceous Seaway retreats Colorado during the deposits as Rockies continue to rise and sea eastward as Present Day retreats eastward. Rockies begin to rise as late Cretaceous Period Laramide Orogeny begins (~70 mya) Fox Hills Sandstone: shoreline deposits as rising Rockies begin to expel the sea. about 70 mya Pierre Shale: thick (~8,000’!), W deep-water marine deposits from E peak of Cretaceous Seaway (see Great Unconformity: map below). marking about a billion years Niobrara Fm.: limestones and of missing geologic time. chalks indicate deeper water Ancient Metamorphic and conditions. Igneous Basement Rocks: Laramide Uplift Benton Fm.: offshore muds and about 1.7-1.4 billion years old, limestones indicate deepening water the roots of an even older as Cretaceous Seaway expands. mountain range. Dakota Group Morrison Fm.. W Lykins Fm. ~65-40 mya: uplift of Lyons Sandstone E Fountain Fm. Rocky Mtns. folds and (discussed in more detail below) fractures overlying layers and begins removing Continued them through erosion Laramide Uplift Cretaceous Seaway ~9-5 mya: W major uplift thrust, via very complex and poorly understood mechanisms, E raises much of western U.S. about a mile into the air! Regional Uplift At its height, the Cretaceous Seaway covered all of what is now Colorado. Dakota Group: alluvial conglomerate beds and beach and lagoonal sands W What we see today at E deposited as the sea once again advances over the land. Heil Valley Ranch Morrison Fm.: mostly terrestrial, lowland swamp, and coastal marsh deposits as the land is eroded nearly flat. ~325 ~225 ~125 mya mya mya Lykins Fm.: mudstones and some dolomitic limestone deposited along the margin of invading shallow seas. Forelle Limestone Member: dolostone containing stromatolites. Forms a slight ridge since it is harder than the surrounding Lykins Fm. “Andesite” (actually Dacite) Igneous Intrusion: Injected as a sill about 63 mya associated with the Laramide Orogeny. Along with Valmont Dike, the youngest rocks in our area! Lyons Sandstone: fine grained, well cemented, cross-bedded coastal dune Great Unconformity sand plain deposits as the sea invades lowland areas of the now extensively eroded mountains. A very useful and popular building stone. Ancient Metamorphic and Igneous Basement Rocks Fountain Fm.: Coarse red sandstones and conglomerates eroded off the recently risen Ancestral Rockies close by to the west. The land was arid and deposits resembled the dry mountain debris aprons and bajadas now seen in Nevada. Read More About It: I suggest the following references that proved very helpful to me in creating this handout: Message In Stone, Colorado’s Colorful Geology, from the Colorado Geological Survey, edited by Matthews, KellerLynn, and Fox; ISBN 1-884216-59-5. Geology Underfoot Along Colorado’s Front Range, by Lon Abbott and Terri Cook; ISBN 978-0-87842-595-2. The Geology of Boulder County, by Raymond Bridge; ISBN 0-9748010-0-3. Ancient Denvers, from the Denver Museum of Nature and Science, by Kirk R. Johnson and Robert G. Raynolds; ISBN 1-55591-554-X. Rocks Above The Clouds, by Jack Reed and Gene Ellis; ISBN 978-0-9760525-8-6. Updated by Roger Myers, October 2014, with thanks to Dick Pratt, Barbara Mieras EchoHawk, Sue Hirschfeld, and Jack Reed for their insights and interpretations. Sedimentary Rocks of the Boulder Area Boulder Thickness Age Stratigraphic Units feet Lithologies Features and Fossils Relative Thicknesses Area (meters) Landmarks River sands 12,640 ft (3853 m) Overbank mudstones with Kl Marshall plant fossils Triceratop s Laramie 750 Coal (swamp deposits) Formation 12,000 (229) Sandstone (Kl) ft Kfh Ranch Estuary sands with oyster beds palm Fox Hills 500 11,000 Sandstone (152) Beach sands Longmont (Kfh) Alternating shales and sandstones 3750 ft gap Laramier and Rocky Ridge Offshore sands 10,000 Ss Members Baculites Locally abundant ammonoid fossils, rare marine lizard (mosasaur) and fish fossils 9000 ) s Terry Ss Mbr 0ffshore sand 45 heteromorph 4 ammonoids eou (2 0 tac re Pierre Shale 02 8000 ic 8 C (Kp) offshore mud deposits ozo that became black shales Kp Mes Offshore sands Boulder Valley Hygiene 7000 that became Open Space Sandstone brown sandstones mosasaur Member Boulder (Kph) Western Interior Seaway Platyceramus 1375 ft gap 6000 Black shales Giant, “dinner plate” inoceramid a clams Smoky Hill 360 foraminifers Member (110) Nelson obrar Kph Road Ni Fm. (Kn) Chalks and black shales Fort Hays Mbr 20 (6) Limestone ledges with bowl- Carlile Mbr shaped inoceramid clams 5000 ) Greenhorn Cremnoceramus n Fm 450 Limestone beds with wing- Kb to Member ( n (137) shaped inoceramid clams Graneros Be Member Offshore muds Mytiloides South Platte 250 ) Formation (76) River to beach deposits dinosaur Kd ( tracks Group 4000 Dakota Lytle Ss 70 (21) Major hogback c Morrison 300 (Jm) Dinosaur bones assi Formation (91) ur Multicolored mudstones J Sundance Ss 30 (9) Apatosaurus dune sand Kn c Six-Mile Lykins si 3000 Fold Formation Deep red mudstones and sandstones as ri (TR Pl) 675 Kb T (206) Rabbit Forelle Ls Mbr Dolostone with stromatolites Kd Mountain Red mudstones ian Jm Lyons Sandstone 220 Dune sands with 2000 (Ply) Major hogback Js Eldorado erm (67) amphibian tracks P Springs ic Fine-grained sandstone sheets Heil Valley TR Pl Ranch ozo le Lyons Fountain Reddish arkosic sandstone Ply Pa nian Formation 1000 and conglomerate derived 1000 va (305) l (PPI f) from the Ancestral Rockies nsy Flatirons n Ancestral Rocky Mountains PPfI Pe Great Unconformity, with a weathered zone 0 Hall Ranch Boulder Creek Granodiorite, Sandstone crack fills (Cambrian?) Boulder t ciozoreorP gneiss, quartzite (pC) ft pC Left Hand Roots of mountains, 1.7 billion years old Creek Canyon Canyon Compiled by E. Evano, 2001, with minor additions by Roger Myers, 2012.