Lecture 1 (Introduction to Stratigraphy).pdf

GEOLOGY 120 Stratigraphy and Sedimentology 1st Semester AY 2020-2021 FB/Messenger Allan Gil Fernando E-mail addresses [email protected]; [email protected] CP number 0917-8404677 ...

GEOLOGY 120 Stratigraphy and Sedimentology 1st Semester AY 2020-2021 FB/Messenger Allan Gil Fernando E-mail addresses [email protected]; [email protected] CP number 0917-8404677  Photo by AGS Fernando What is stratigraphy? The science of rock strata. It is concerned What is sedimentology? not only with the original succession and age relations of rock strata but also with Sedimentology is the study of the their form, distribution, lithologic processes of formation, transport and composition, fossil content, geophysical deposition of material that accumulates and geochemical properties, with all as sediment in continental and marine characters and attributes of rocks as strata environments and eventually forms and their interpretation in terms of sedimentary rocks. environment, mode of origin and geologic history. Nichols (2009) not restricted with sedimentary rocks, strat can be formed with other rocks (ign and met) Note: All rocks, consolidated or not, fall within the scope of stratigraphy AGI definition  Why is Stratigraphy Important to Geologists? 1. For geological investigation (stratigraphic framework); 2. Correlation purposes; deposition at the same time 3. Paleoenvironmental interpretation; how the environment change thru time (lithology, sed structure, fossils) 4. Reconstruction of Earth’s history based on relationship of rocks STRATIGRAPHY SUBFIELDS** (not a complete list) Lithostratigraphy rock types Biostratigraphy fossils Magnetostratigraphy magnetic reversal time scale Chemostratigraphy chemical or isotopic methods for correlating Chronostratigraphy (lecture on GTS)  HISTORICAL PERSPECTIVE ON THE ORIGINS OF STRATIGRAPHY d’ Orbigny (1852) stratigraphie - founding father of paleo/micropaleo(?) stratum and graphia = (descriptive writing about) or the study of strata or stratified rocks Guiding Principles in Stratigraphy 1. Steno’s Laws (Nicolaus Steno/Niels Steensen; 1638 – 1686) A. Original Horizontality B. Original Continuity C. Superposition De solido intra solidum naturaliter dissertationis prodromus  Original Horizontality Original statement: As regards form, it is certain that at the time when any given stratum was being produced, its lower surface, as also its lateral surfaces correspond to the surfaces of the lower substance and lateral substances, but that the upper surface was parallel to the horizon, so far as possible; and that all strata, therefore, except the lowest, were bounded by two planes parallel to the horizon. Hence, it follows that strata either perpendicular to the horizon or inclined toward it were at one time parallel to the horizon. Modified Statement: Law of Horizontality: Sedimentary strata are laid down nearly horizontally and are essentially parallel to the surface upon which they accumulate Keyword: parallel to the surface of deposition * Deposition can be possible even though the surface is inclined - angle of repose of the slope must not exceed the angle of repose of the sediments - Angle of repose -> moisture content, angularity, and grain size QUESTION 1: What is the angle of repose and what factors control the angle of repose of the sediments?  Original Continuity Original statement: At the time when ay given stratum was being formed it was either encompassed on its sides by another solid substance, or it covered the entire spherical surface of the earth. Hence it follows that in whatever place the bared sides of the strata are seen, either a continuation of the same strata must be sought, or another solid substance must be found which kept the matter of the strata from dispersion. Modified Statement: Law of Original Continuity: The original continuity of water laid sedimentary strata is terminated only by pinching out against the basin of deposition at the time of their deposition and may grade into other rock types as a result of facies change. *Disclaimer (pagintindi ko): deposition of the sediments/rocks must be continuous along/within the basin Discontinuous layers that are similar on either side of a valley must have been separated by erosion from their original continuous state.  Facies (Walther, 1893-1894) http://www.thisoldearth.net/Geology_Online-1_Subchapters.cfm?Chapter=6&Row=2 A body of rock characterized by a particular combination of lithology, physical and biological structures that bestow an aspect (“facies”) different from the bodies of rock above, below and laterally adjacent Facies change due to the following: 1.) Depends on the distance from the deposition area 2.) Differences in the energy 3.) Differences of fossil assemblage in the area Walker and James, 1992 http://www.thisoldearth.net/Geology_Online- 1_Subchapters.cfm?Chapter=6&Row=4 Lithologic Correlation – equivalency of rock units across an area AGS Fernando et al. (2010) Geobios Chemostratigraphic and Biostratigraphic Correlation  Superposition Original statement: At the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lowest stratum was being formed, none of the upper strata existed. Modified Statement: Law of Superposition: The youngest strata are at the top in an undisturbed sequence of sedimentary rocks. Review: Criteria for determining the top and bottom of beds (AGI Data Sheet section 10.1)  Criteria for Determining Top and Bottom of Beds: 1. Physical a. Tracing of beds or recognition of a known normal sequence b. Scouring or channeling and (basal) conglomerates erosive activity c. Solution surfaces karstic envi/limestones d. Graded bedding e. Cross bedding f. Mud cracks exposed to dessication g. Ripple marks h. Sole marks 2. Paleontological exp. ichnofossils, corals (limestones -> acient coral reefs as evidence -> upright corallites) 3. Physical criteria for igneous rocks (lava flows) a. Presence of vesicles b. Contact metamorphism cold rocks had contact with the hot rocks exp. lava flow c. Cusps (pillow lavas) kung saan nagpopoint ung cusp = dalom ; rounded part of the pillow = top d. Columnar jointing cooling of the top of the lava flows Photo by AGS Fernando  Photo by AGS Fernando QUESTION 2: What sedimentary structures can you see in the picture that can be used as top and bottom bed indicators  Photo by AGS Fernando QUESTION 3: Can you give an explanation or several explanations what happened to this outcrop?  Photo by AGS Fernando QUESTION 4: What sedimentary structure is shown here? Explain how this structure can be used as top and bottom bed indicators.  Photo by AGS Fernando QUESTION 5: What sedimentary structure is shown here? Explain how this structure can be used as top and bottom bed indicators.  Photo by Hisatake Okada QUESTION 6: Shown here is a lava flow in Hokkaido, Japan. What features or structures can be used as top and bottom indicators of lava flows?  QUESTION 7: This is an outcrop showing an igneous unit “sandwiched” between sedimentary layers. How will you determine whether it is a sill or a lava flow? What evidence should you look for? Photo by AGS Fernando  Other Guiding Principles in Stratigraphy: D. Principle of Cross- Cutting Relationship Applies to geologic features like faults, or intrusions of igneous rock or veins of minerals, or ancient erosion surfaces called unconformities that cut across pre-existing rocks. -> they are younger than the rocks that they cut Photo by AGS Fernando  Other Guiding Principles in Stratigraphy: E. Principle of Inclusions -> they are always older than the rocks surrounding them There are two fundamentally different types of inclusions. 1. Inclusions in sedimentary rocks - the gravel clasts in the layer above an unconformity. 2. Inclusions in igneous rocks - A xenolith is a fragment of the surrounding rock which has broken off during an intrusion and fallen into the magma. Photo by AGS Fernando  Photo by AGS Fernando Igneous intrusion (dike) in Catanduanes Island. Take note of the sedimentary layers on the left side of the picture.  Photo by AGS Fernando Inclusions of large clasts of mudstones derived from the erosion of the underlying unit during the deposition of the coarser unit above it. These large clasts are sometimes referred to as “rip up clasts” -> eroded and deposited w/ younger surrounding seds Other Guiding Principles in Stratigraphy: F. Uniformitarianism (James Hutton; 1727-1797) - “the present is the key to the past” G. Faunal Succession (William Smith; 1769-1839) - the observed chronologic sequence of life forms through geologic time (i.e., fossil organisms succeed one another in a definite and recognizable order, each geologic formation having a different total aspect of life from that in the formations above it and below it; useful in relative dating techniques). - used by Charles Darwin as part of his proof for the theory of organic evolution. Other Guiding Principles in Stratigraphy: H. Unconformities - Temporal break in a stratigraphic sequence resulting from a change in regimen that caused deposition to cease for a considerable span of time. It normally implies uplift and erosion with the loss of some of the previously formed record (Dunbar and Rogers, 1957). -- surfaces due to erosion/non-deposition time is missing/ geologic record is broken - The term unconformity is restricted to apply only to the actual rock surface or interface between vertically adjacent strata. - Nonconformity ign/met unit (eroded/weathered) is older than the sed unit (find erosional surface) *Diastem- minor erosion/non-deposition (exp.bedding planes) - Disconformity erosional surface is obvious *Hiatus- time represented by the unconformities or diastem - Paraconformity erosional surface is not obvious *Unconformities and diastem are physical manifestation of missing sed record. - Angular Unconformity beds below the unconformity are tilted while the beds at the top are horizontal - applies to the actual rock surface/interface bet. vertically adjacent strata. QUESTION 8: Differentiate unconformity, hiatus and diastem  This is an outcrop in Batanes showing a lower sandstone unit and an upper limestone unit. What do you notice about the contact between the two layers? Question 9: What kind of unconformity is this? Photo by AGS Fernando This is another outcrop in Batanes showing a lower igneous unit and an upper limestone unit. Question 10: What kind of unconformity is this? Photo by AGS Fernando  This is an outcrop in Damortis, La Union showing a lower siltstone unit, and an upper unit consisting of coral rubble. Question 11: What kind of unconformity is this? Photo by AGS Fernando  Photo by AGS Fernando This is an outcrop in Kalinga showing a tilted lower gray mudstone unit that is overlain by subhorizontal sandstone and conglomerate units. The obvious discordance in the dip of the lower and upper units, and the obvious erosional surface suggest an angular unconformity.  Criteria for Recognition of unconformities: 1. Sedimentary a. Basal conglomerate clasts that undergo weathering; erosive envi. b. Lag gravel (eolian environments) cover of course seds on top of the sand carried by wind (hydrous potassium) c. Glauconite zones *Glauconite- greenish mineral non-deposition (very slow sedimentation); clastic + glauconite; greenish sandstone d. Iron oxide zones non-oxidizing seds that were exposed to oxidizing conditions; subaerial conditions was originally situated under H2O but was exposed to air e. Interbedded conglomerates alternating units; changes of regime due to influx of energy f. Clastic zones in non-clastics exp. reefal envi can die when malapit sa mouth of the river (input of seds) drastic shift/change in the envi g. Sharp differences in lithology above and below the contact h. Buried soil profiles exposed to weathering agents 2. Paleontologic a. Abrupt changes in fauna drastic change in the environment (lost sed record); missing flora/fauna b. Gaps in evolutionary development microfossils missing in the seq of events c. Borings of littoral marine organism - borings/ boundary ng H2O and coastline subaqeous to subaerial 3. Structural a. Discordance of dip above and below a contact b. Undulatory surface of contact c. Truncated dikes intruded by dikes; there’s a sharp change that the dike does not continue in the upper layer/unit