Stratigraphic Contacts

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1 Stratigraphic Contacts

2 I. Intro A. Litho. unit separated by contacts (planar or irregular surface) B. Contacts conformable or unconformable D. Conformable= unbroken depositional sequence--mostly layers deposited under similar conditions E. Surface separating conformable strata=conformity; surface shows no evidence of non-deposition F. Therefore, conformable contact= no significant break or hiatus in deposition

3 Conformable Contact Upper part makes a gradational transition from St. Peter Sandstone into overlying Joachim Dolomite (thru dolomitic sandstone, sandy dolomite) suggesting it is a conformable formation contact

4 Conformable Contact Silurian Tuscarora Sandstone and darker Fm.,Virginia

5 Intro Continued F. Hiatus = interval of geol. time represented by missing strata G. Unconformable strata do not succeed rks in age or do not fit to form continuous sequence H. Contacts between unconformable strata= unconformities I. Unconformity = surface of erosion or non-deposition; separate younger rks from older rks 1. unconformity represents significant hiatus J. Unconformity= lack of continuity in deposition; corresponds to periods of non-deposition, weathering or erosion prior to deposition of younger beds

6 II. Contacts between conformable strata
A. Contacts between strata abrupt or gradational B. abrupt/sharp= sudden distinct changes in lithology C. bedding planes are abrupt contacts= short depositional breaks= diastems D. abrupt contacts also formed by diagenetic changes e.g. color, cement E. gradational contact - gradual change form one lithology to the other= gradual change in depositional conditions with time F. progressive gradual contact, one lithology grades into the other G. intercalated = gradational contact w/increasing interbeds of upsection lithology

7 If this sediment surface is subjected to the erosive force of:
Beds are enclosed or bounded by sharply defined upper and lower surfaces or bedding planes. These surfaces are probably the easiest physical features of sedimentary rocks to identify in outcrop. They are used to subdivide successions of sedimentary rock into their beds and are traditionally used to determine the relative order and timing of the accumulation of the sediments forming the beds. Concurrently the character of the bedding planes be they eroded, cemented, bored, bioturbated, or depositional surfaces is used to aid in the interpretation of these sedimentary rocks. To this end Allen (1983) has described, using fluviatile sediments as an example, that there at least four kinds of bounding sufaces: concordant non-erosional (normal bedding) ; disconcordant non-erosional ; concordant erosional; and disconcordant erosional contacts. Never the less, the origin of the bedding plane can be enigmatic. The hypothesis presented here is that most of these bedding planes are probably surfaces formed by the erosion of unconsolidated sediment that collected at the sediment surface. The weight of the sediment, just beneath the sediment surface, causes this sediment to dewater, compact and become cohesive. If this sediment surface is subjected to the erosive force of: • Storm waves • Fast flowing currents of water (say in tidal or fluvial channels) • Turbid flow of a density current then the bedding plane surface will cut down into sediment, truncating the less cohesive sediment of the surface and exposing a surface of the firmer cohesive sediment below. strata.geol.sc.edu/terminology/beddingplane.html

8 Sharp Contacts Exer[ted from: Did Modern Coal Seams Form in a Peat Swamp? by John D. Morris, Ph.D. The accompanying photograph shows two coal seams visible in a road cut near Price, Utah. Observe the sharp contacts between the coal and the adjacent layers. If a swamp existed above sea level, but then was inundated by the sea to receive overlying marine sediments, then uplifted to become a swamp again, and the cycle repeated, one would think there would be some erosional channels or variation in peat thickness? How could they be so flat and of constant thickness, and how could there be such precise contacts above and below? Modern-day peats do not seem to be of the same character as modern-day coals. Perhaps the creation model of decaying plant material collecting under a large floating mat during the Great Flood of Noah's day provides a better answer. This article was originally published August, "Did Modern Coal Seams Form in a Peat Swamp?", Institute for Creation Research, (accessed April 06, 2009).

9 Rushford Fm., sharp, erosive contact between the transgressive lag (displaying steeply dipping cross-beds) and the finer-grained shoreface sandstones at roadcut

10 Knife-sharp contact between Olentangy (below) and Huron Shale (above)
Knife-sharp contact between Olentangy (below) and Huron Shale (above).  Stream bed, Columbus/Ohio

11 Well-developed flooding surface in carbonate strata
Well-developed flooding surface in carbonate strata. Note the sharp contact between dissimilar facies at the hammer head, with shallow subtidal facies below the hammer and deep subtidal above. This photography was taken in the Middle Ordovician Chickamauga Group near Fort Oglethorpe, Georgia.

12 Interbedding of chert and sandstone, observed in the study area as well as in other parts of the Franciscan, requires special depositional processes and environments. For example, chert in the Marin Headlands terrane accumulated at rates of about 0.3 mm/ka to 2.5 mm/ka (Murchey and Jones, 1984; Hagstrum and Murchey, 1993). In contrast, turbidites on the modern Madeira Abyssal Plain can accumulate at rates of about mm/ka (Stow et al., 1996). Even the fastest rate of chert sedimentation would require 60 ka to produce the 15-cm-thick chert bed described in the study area. It is highly unlikely that so much time passed between turbidity currents in the thickly sedimented Franciscan subduction zone. Thus, the interbedding probably occurred at the most distal edge of a fan near the abyssal plain, in a high-productivity zone such as a marginal basin, or due to pelagic turbidity currents generated by instability upon a bathymetric high.

13 Sharp contact at the top and gradational contact at the base of Facies II-4 clay (interval A-5H-3, cm). www-odp.tamu.edu

14 Gradational? Rushford � Caneadea contact at outcrop along Hillcrest Rd., Houghton quadrangle.

15 III. Contacts between unconformable contacts
A. 4 types of unconformities 1. Angular unconformity 2. Disconformity 3. Paraconformity 4. Nonconformity

16 III. Contacts between unconformable contacts
B. Angular unconformity= younger sed on eroded surface of tilted or folded rks 1. have local and regional angular unconformities C. Disconformity= parallel bedding above & below irregular & uneven erosional surface 1.disconformity may be identified by gravel lag or soil horizon 2.disconformity = period of erosion in which older rks remained parallel during uplift D. Paraconformity = beds above & below unconformity contact are parallel--don’t see erosional or other physical evidence of unconfomity 1. paraunconfomity identified through fossil data C. Nonconformity-unconformity between sed rks & older ign or meta rks that were exposed prior to being covered by sediments E. Unconformity indicate major geologic event took place

17 Horizontal fluvial deposits on truncated dipping beds call attention to this angular unconformity at Cody Wyoming. Copyright © Bruce Molnia, Terra Photographics

18 Copyright © Marli Miller, University of Oregon

19 Copyright © Marli Miller, University of Oregon
Angular unconformity at Siccar Point. It is here, at this famous unconformity, that James Hutton made many of his observations leading to modern thinking in the geosciences. This spot is also knows as Hutton's Unconformity.

20 Disconformity near Canmore, Alberta Devonian rocks lie directly on rocks of the Cambrian period.
Copyright © Bruce Molnia, Terra Photographics

21 Disconformity - unconformity, or gap in geologic time, caused by the process of erosion
Copyright © Marli Miller, University of Oregon

22 Nonconformity between the Cambrian Tintic Quartzite on Precambrian basement.
Copyright © Marli Miller, University of Oregon

23 Unconformity with crystalline granite in contact with Mesozoic sandstone occurs near Colorado Springs. Copyright © Bruce Molnia, Terra Photographics

24 IV. Vertical Succession of Beds
A. beds succeed each other vertically in ways such as B. lithologic uniformity C. lithologic heterogeneity D. cyclic successions

25 Buttress angular unconformity in northern Arizona
Buttress angular unconformity in northern Arizona. A buttress unconformity is one in which the younger, overlying rocks are cut by the contact. This relationship occurs because the younger sediments are deposited against the older rocks as they stood out in topographic relief

26 Gravel Lag? Unconformity on the Langness peninsula, IOM
The lower part of the conglomerate contained angular and poorly sorted pebbles from the Manx Group.

27 Contacts between laterally adjacent lithosomes
pinchouts intertonguing progressive lateral gradation