Harry Williams, Historical Geology LECTURE 1. STRATIGRAPHY Stratigraphy: Many past geologic events have resulted in strata or layers of sediment which have formed a stacked vertical sequence of sedimentary rocks. A good example is sediment in the form of mud and sand from a river accumulating on a sea-floor. The sediment forms a layer which eventually becomes hardened or lithified into a sedimentary rock. Harry Williams, Historical Geology

Harry Williams, Historical Geology The importance of stratigraphic sequences to historical geology is that they provide a rock record of past events e.g. a pebbly beach may be recorded as a CONGLOMERATE, a sandy river may be recorded as a layer of SANDSTONE; a clayey tidal flat as a layer of SHALE. Pebbly beach conglomerate Harry Williams, Historical Geology

Harry Williams, Historical Geology Sandy beach sandstone Harry Williams, Historical Geology

Harry Williams, Historical Geology Muddy marsh shale Harry Williams, Historical Geology

Harry Williams, Historical Geology Stratigraphy is the study of these layered rocks, especially their sequence, correlation from place to place, relative ages and interpretation. Several important stratigraphic principles emerged from the study of stratigraphy centuries ago by the early founders of the science of geology: Harry Williams, Historical Geology

Harry Williams, Historical Geology NICOLAUS STENO (1669) 1. Principle of Superposition: In any sequence of undisturbed strata, the oldest layer is at the bottom and higher layers are successively younger. Harry Williams, Historical Geology

Harry Williams, Historical Geology Connected to this is the idea of conformable and unconformable sequences. A conformable sequence is one in which there has been continuous deposition of sediment such that the resulting rocks grade one into another via gradational contacts. An unconformable sequence is one in which deposition has been interrupted by periods of erosion, causing removal of some layers and resulting in erosional contacts. The unconformity represents a GAP in the rock record. Various types of unconformities result, including angular unconformities (sedimentary strata meet at an angle), disconformities (essentially parallel strata) and nonconformities (younger strata on top of older or younger metamorphic/igneous rocks). Harry Williams, Historical Geology

Harry Williams, Historical Geology

Harry Williams, Historical Geology Continuous deposition of sediment layers. Uplift and folding of conformable sequence of sedimentary rocks. Erosion forms an erosion surface. Renewed deposition forms an angular unconformity. Harry Williams, Historical Geology

Harry Williams, Historical Geology 2. Principle of Original Horizontality: Most sedimentary rocks formed originally in close-to-horizontal layers (although many have since been moved from their original position). Undeformed strata Deformed strata (tilted) Harry Williams, Historical Geology

Harry Williams, Historical Geology 3. Principle of Original Lateral Continuity: Originally sedimentary strata extended in all directions until they either; thinned out, ended abruptly at some kind of barrier or graded into a different kind of sedimentary rock. This principle is important for correlating sedimentary rocks from place to place i.e. across a valley. Stratigraphic correlation is the practice of "matching up" equivalent bodies of rock from different locations. The equivalence may be in terms of lithology, age or fossil content. Harry Williams, Historical Geology

Harry Williams, Historical Geology

Harry Williams, Historical Geology JAMES HUTTON (1726 - 1797) 4. Principle of Uniformitarianism: "The present is the key to the past" i.e. geologic processes operating today also operated in the past. e.g. river deposits forming today have a similar composition and character as their ancient counterparts; glacial erosional and depositional features are basically the same today as in the past...etc. Modern sand dune Ancient sand dune Harry Williams, Historical Geology

Harry Williams, Historical Geology

Harry Williams, Historical Geology WILLIAM "STRATA" SMITH (1769 - 1839) 5. Principle of Biological Succession: Different kinds of plants and animals succeed one another in time because life has evolved continuously; therefore only rocks formed during the same age can contain similar assemblages of fossils. Since these fossil assemblages are unique for particular periods of the past, they can be used to: a. correlate rocks from around the world, and to b. order rock layers into a sequence of relative age (i.e. older …. newer). Harry Williams, Historical Geology

Harry Williams, Historical Geology CHARLES LYELL (1830) 6. Principle of Cross-Cutting Relationships: Any geologic feature which cuts across or penetrates another body of rock must be younger than the rock mass penetrated. Harry Williams, Historical Geology

Harry Williams, Historical Geology Sandstone Fault A Conglomerate Shale Fault B Batholith Dyke B & sill Dyke A Older Younger Harry Williams, Historical Geology

Harry Williams, Historical Geology 7. Principle of Inclusions: Any rock that contains fragments of an adjacent rock must be younger than the adjacent rock. Harry Williams, Historical Geology

Harry Williams, Historical Geology What was the sequence of events? Harry Williams, Historical Geology

Harry Williams, Historical Geology

Harry Williams, Historical Geology

Harry Williams, Historical Geology