#sedimentary structure

Ancient Ripples

These layers are pretty neat when you can find them. This is a surface in the Tradewater Formation of Southern Illinois covered with ripples.

Ripples like these are characteristic of the edge of ocean basins or lakes, where wave action and water currents can move the sediment around. Preserving the ripple marks requires covering them without destroying them – an event like a storm that brings a lot of extra sediment can bury previously-formed ripples without destroying them.

Finding these ripple marks establishes that the sediment was deposited near a shoreline, in a setting with active currents. The Tradewater formation is Pennsylvanian in age, formed about 350 million years ago when much of Central North America was submerged in a shallow ocean. The formation also has a number of coal beds within it, formed as the seashore migrated in and out as sea level changed.

Ripples like these can be useful for a geologist. If rocks have been folded it can be difficult to tell which side of a unit was the top – ripple marks only form on the top of a unit and so can be used to say which direction was “up” (although there can be casts of the ripples formed from the sediments that touch these ripples). Ripple marks can sometimes even tell where currents were flowing, how strong currents were, and how deep the water was.

-JBB

Image credit: https://flic.kr/p/ozbh9N

中期中新世（約1200万年前）の深海底に生息した生物の巣穴の化石

地層に残された大きなウェーブリップル（写真中央の横に広がる茶色い波模様）．茨城県行方市玉造．

＜もう少し説明＞

更新統下総層群中に見られる大型のウェーブリップル（波長約1 m）で，粗粒砂から細礫の粗い堆積物で構成されている．Coarse-grained ripple（Leckie, 1988）と呼ばれるものに該当する．その大きさから「ウェーブデューン」とも記載されていた（牧野・増田, 1986）．表面は泥層に覆われている．

Here's an example of a sedimentary structure called a sheet cavity! The grey-white lines within the yellow dolomite matrix. These structures are diagnostic of ancient reef-structures, and the calcareous composition further points towards reef-related beginnings.