8020–8024 | PNAS | May 14, 2013 | vol. 110 | no. 20 pnas/cgi/doi/10.1073/pnas.1221965110

1. 8020–8024 | PNAS | May 14, 2013 | vol. 110 | no. 20 www.pnas.org/cgi/doi/10.1073/pnas.1221965110 Nanoscale analysis of pyritized microfossils reveals differential heterotrophic consumption in the ∼1.9-Ga Gunflint chert Fig. 1. Occurrence of pyritic microfossils at Schreiber Channel. (A) Stromatolitic chert with microfossil-rich laminae. Pyritic microfossils occur most commonly in millimeter-sized patches surrounded by clear chert (circled). These patches frequently pass laterally into areas rich in organic material and carbonaceous microfossils. (B) Laser Raman map (Inset) showing filamentous sheaths of Gunflintia that are part carbon (red) and part pyrite (green). Harry Williams, Historical Geology

2. HISTORICAL GEOLOGY LECTURE 8. EARLY PALEOZOIC GEOLOGY I. Introduction The paleozoic is divided into an early and late period. Cambrian + Ordovician + Silurian = early paleozoic (126 m.y) Devonian + Mississippian + Pennsylvanian + Permian = late paleozoic (165 m.y). Harry Williams, Historical Geology 2

3. Both these "periods" have a similar geologic history characterized mainly by large scale transgressions, orogenic events and regressions. Each period can be examined in terms of the major events that shaped the geologic record - these events are: 1. Paleogeography 2. Transgressions and Regressions 3. Orogenies Harry Williams, Historical Geology 3

4. 1. Paleogeography: Location of land and sea areas. Land - often erosion, non deposition; exception = arkosic sandstone + volcanics. Sea (including epeiric seas) - deposition of clastics and carbonates. Paleolatitude - refers to the location of the North American craton in relationship to the Equator and Poles. Paleoclimate Rock types e.g. warm/hot climates -> carbonates, coral reefs, evaporites. cold climates -> glacial deposits. Harry Williams, Historical Geology 4

5. 2. Transgressions and Regressions Occurred mainly because of tectonic plate movements. TRANSGRESSION -> shallow ocean in PLATFORM areas ("Epeiric seas" - sea over a continent), which underwent DEPOSITION. REGRESSION -> land exposed - usually underwent EROSION (with some exceptions including arkosic sandstones). Because marine conditions often meant deposition and terrestrial conditions often meant erosion, a major transgression followed by a regression created a sequence of strata capped by an UNCONFORMITY. (on board) Harry Williams, Historical Geology 5

6. The green zone represents a cratonic sequence (rocks laid down during a transgression and regression). The lower boundary of the green zone is a transgression; the upper boundary is a regression. Harry Williams, Historical Geology 6

7. The sequence is thicker and more complete near the craton edges since these areas are the first to be covered by the transgressing sea and the last to be uncovered by the regressing sea. The Paleozoic contains a number of these major depositional sequences - the SAUK in Cambrian time and the TIPPECANOE in Ordovician/Silurian time. Orogenies Mostly resulting from plate convergence and continent-continent collisions. Mountain chains were constructed along craton margins, consisting of uplifted, folded, metamorphosed marine and terrestrial deposits - often accompanied by intrusive and extrusive igneous rocks. These mountain chains were subject to erosion and became sediment sources for later deposits - shales + greywackes in deep marine basins; quartz sands and shales in shallow platform seas; fluvial sandstones and conglomerates in terrestrial areas. Harry Williams, Historical Geology 7

8. 1. Paleogeography 750 million years ago Late Cambrian (514 million years ago): Rodinia had broken up. The major land masses (Laurentia, Baltica, Kazakhstania, Siberia, China and Gondwana) were moving apart. 8

9. The North American craton lay along the Equator during Cambrian time (Mexico - Arctic Canada). Harry Williams, Historical Geology

10. CAMBRIAN 1. Paleogeography The North American craton lay along the Equator during Cambrian time (Mexico - Arctic Canada). The climate was warm/hot. The craton land area was towards the northeast and eroding. Shallow platform seas existed around the margins of the craton, especially in the south. The deposits forming in these seas were gradational from sand-shale-carbonate, depending on depth of water. The break-up of Rodinia had crated linear marine basins (seaways) around the edge of the craton: Appalachian seaway to the S.E./E. Cordilleran seaway to the W. Franklin seaway to the N. These were sites for SHALE deposition. Harry Williams, Historical Geology 10

11. 2. Transgressions/Regressions The Cambrian was a time of TRANSGRESSION - the SAUK TRANSGRESSION. As the coastline moved inland, a typical transgressive sequence was deposited: COASTAL SANDS SHALLOW MARINE SHALES DEEPER MARINE LIMESTONES A good example is provided by the Cambrian deposits exposed in the Grand Canyon: Old at bottom Young on top Harry Williams, Historical Geology 11

12. Transgressivesequence: TEPEATS SANDSTONE (coastal sand) BRIGHT ANGEL SHALE (shallow marine shale) MAUV LIMESTONE (deeper marine limestone) Harry Williams, Historical Geology 12

13. O: lower Cambrian. G: middle Cambrian. The sequence records the transgression and the migration of the coastline from west to east.

14. Due to the slow progress of the transgression, the same sequence did not develop everywhere at the same time. E.g. Bright Angel Shale is lower Cambrian in California; middle Cambrian in the Grand Canyon. This is an example of a “Diachronous Rock Unit” (a layer of rock of non-uniform age). Above: one layer of sandstone; younger to the left, older to the right. Harry Williams, Historical Geology 14

15. Sedimentary environments near the peak of the Sauk transgression (late Cambrian).

16. The end of the Sauk transgression (Sauk regression) is marked by a widespread erosional unconformity, which in turn is overlain by the Tippecanoe cratonic sequence. The erosion is prominent above an uplift that formed in the craton.

17. Broad warping of the craton formed these basins and domes during and after deposition of these rocks. Other uplifts are related to orogenic activity.

18. Tepeats sandstone in the Grand Canyon records lower Cambrian Suak transgressive sands. St. Peter sandstone records middle Ordovician, Tippecanoe transgressive sands.

19. Are there any lower Paleozoic rocks around DFW? Yes, but they are west of Denton and they are upper Paleozoic (Pennsylvanian and Permian). They are in the Fort Worth Basin. Lower Paleozoic rocks are underneath them and are exposed around the Llano Uplift in the Hill Country. Harry Williams, Historical Geology

20. Section through the western part of the Fort Worth Basin Harry Williams, Historical Geology

21. 3. Orogenies Harry Williams, Historical Geology 21

22. 3. Orogenies The Cambrian was a stable period for North America. The fragments of Proto-pangaea were moving APART, so the edges of the craton were PASSIVE MARGINS -> no convergence; no subduction; no orogenies. Harry Williams, Historical Geology 22

23. Seaways formed at the craton edges; these were sites for coastal sands and shales, shallow water carbonates and deeper marine shales offshore. E.g. a thick shallow water carbonate layer formed along the Appalachian Seaway from Newfoundland to Alabama in Cambrian time. Harry Williams, Historical Geology 23