Rocks of the Inland Basin

Sedimentary rocks dominate the Inland basin because the area was covered by the ocean for tens of million of years: first in the Cambrian when global sea level was high and the ocean stretched far inland over most of the Northeast, and later during the Taconic and Acadian mountain-building periods (Ordovician through Devonian) when an inland ocean existed west of the new mountain ranges. The basin of the inland sea formed by the buckling of the crust from the compression of plates during the mountain-building events. Conglomerates, sandstones, siltstones, shales, limestones and dolostones are common rocks formed in these oceans and the bordering environments such as deltas, swamps, mud flats and tidal areas.

Why are there different sedimentary rocks in different environments?

As mountainous highlands, erode, sediments are transported down the mountain by gravity and streams. The sediments that have only been transported a short distance and have not undergone considerable weathering, form conglomerates when compacted and cemented. Conglomerates are made of poorly sorted sediments, containing large pebbles of rock as well as finer sediments in between, typical for a deposit that occurs close to the source of erosion. If the sediments are transported a bit farther before being deposited and undergo more wearing down along the way, the sediments become rounded, smaller, and better sorted, as all of the larger grains drop out of the slowing water. If you examine sediments form the beach out to deep ocean, you will notice that beach deposits (and river deposits) are mainly sandy, followed by finer grained silts in deeper water, and very fine-grained clay in the deepest water above which currents may be slow enough to permit such small particles to settle. Limestones tend to accumulate where the rate of sediment being eroded from the highlands is low enough not to dilute accumulation of the calcium carbonate shell material that forms limestone. Many organisms that secrete calcium carbonate shells thrive ecologically in the clear water. In a shallow continental sea, low rates of sedimentation and clear water may be reached further from shore where sediment derived from land has settled out. Thus, the typical sequence of rocks formed across a shallow continental basin at any give time begins with conglomerate near the source, and sandstone, siltstone, shale and reefy limestone forming farther out. Figure by J. Houghton.

Precambrian Adirondack Rocks

Figure 2.2: Precambrian Adirondack rocks exposed in the Inland Basin.

The Adirondack region of New York is composed of many types of billion-year-old rocks, all of which were metamorphosed as a result of the Grenville Orogeny. These ancient metamorphosed rocks are an anomaly in the basin region, which is dominated by sedimentary rocks. Grenville-aged rocks that were originally sandstones, limestones and shales deposited in a warm, shallow ocean at the eastern margin of proto-North America, make up the bulk of the resistant rocks of the Adirondacks (Figure 2.2). These are the oldest rocks found at the surface in the Northeast. As Baltica approached North America for the first time (in the Late Precambrian), the Grenville belt of sedimentary rocks was squeezed and pushed up onto the margin of proto-North America, forming the Grenville Mountains. During the intensity of the squeeze, the sedimentary rocks were metamorphosed. Sandstone became quartzite, gneiss or schist; limestone became marble; and shale became gneiss and schist.

There are other types of rocks exposed in the Adirondack region as well. During the Grenville mountain-building event, magma created by the friction between the converging plates was rising up into the overlying crust. The blobs of magma rose higher, pushing through overlying sedimentary rocks. The blobs eventually cooled and crystallized, forming igneous rocks such as granite, anorthosite and, less commonly, gabbro. As the Grenville Orogeny continued, the cooled igneous blobs and the sedimentary rocks of the Grenville Belt were buried under as much as 30 kilometers of crust! With that much crust overhead, the pressure and temperature on the buried rocks was extremely high, causing further metamorphism. The granites became gneiss; gabbros became metagabbro; and the anorthosite became metanorthosite. The intensity of the Grenville mountain-building event also sheared the rock as blocks of crust slid past each other in opposite directions. This is most evident in a band of rocks called mylonites in which minerals were compressed and recrystallized upon shearing.

For millions of years following the Grenville mountain-building event, the Grenville Rocks that stretch from Canada to Mexico were worn down and buried by layers of sedimentary rock. Grenville-age rocks are present in many other parts of the Northeast, but are generally deeply buried by younger overlying sedimentary rocks. In the Adirondack region, the Grenville rocks are exposed because of an uplift of the crust that occurred only 10-20 million years ago during the Tertiary period.

The Adirondacks, though composed of billion-year-old rocks, are actually relatively young as mountains. Their exact mode of formation is still debated. Some geologists think that the crust was uplifted because of a hot spot beneath the crust caused by plumes of magma rising from the asthenosphere. As the magma rose, the crust was pushed upwards, forming a dome. The softer sedimentary rocks on top of the dome were uplifted, fractured, and eroded away quickly, exposing the underlying Grenville rocks.

Cambrian-Ordovician Rocks

The remaining rocks exposed in the Inland Basin are sedimentary rocks. As you move from north to south on the geologic map of the basin, you will notice that the exposed surface rocks become younger (Figure 2.3). 

Figure 2.3: The east-west stripes of rocks in the Inland Basin occur because of the shallow angle of the rock layers. Regional compressional stress from mountain building tilted the layers of sedimentary rock gently, less than five degrees to the south. The tilting exposes layers of rock that would otherwise remain buried.

Cambrian and Ordovician rocks are exposed in northernmost New York rocks and in patches around the Adirondack dome, followed by a thin stripe of Silurian rocks to the south. Most of the southern tier of New York and northern Pennsylvania exposes Devonian sedimentary rocks, followed by exposures of Mississippian, Pennsylvanian and Permian rocks continuing south into Maryland. These rocks were at one time flat-lying layers of sedimentary rock, with the Cambrian rocks lying unseen beneath overlying younger rocks. The layers, however, have been tilted very gently a few degrees to the south and eroded, exposing the underlying older rocks.

Figure 2.4: The Taconic volcanic islands approached from the east during the Cambrian. Figure by J. Houghton, after Geological Highway Map of the Northeastern Region, no. 10, 1995.

When the Grenville mountain building finally subsided in the late Precambrian, a period of erosion followed that wore down the ancient Grenville Mountains, which stretched up the margin of North America. During this period, the Iapetus Ocean opened and widened as Baltica separated once again from North America. Rifts developed in the crust during this separation, creating small basins of down-dropped blocks of crust. Cambrian sedimentary rocks from the eroding Grenville highlands are preserved in the rift basins. The rift basins appear in patchy areas around the Adirondack dome. Globally, sea level rose during the late Cambrian, covering most of the Northeast with a shallow ocean (Figure 2.4). Sedimentary rocks formed form the sediment eroded from land to the west. 

Figure 2.5: Cambrian and Ordovician rocks exposed in the Inland Basin. 

Most of the early Ordovician produced similar deposits to the Cambrian (Figure 2.5). Sea level remained high and the rocks formed were predominantly limestone and dolostone, common in warm, shallow, sediment-starved seas. As sea level dropped later in the Ordovician, these sedimentary rocks were subjected to intense erosion.

Towards the end of the Ordovician, volcanic islands that had formed along the subduction zone between North America and Baltica, moved towards the margin of North America. Layers of ash resulting from the volcanic activity to the east were deposited in the basin and can be seen today preserved within the rocks of the Inland Basin. The volcanic islands collided with North America to form the Taconic Mountains and buckled the crust to the west of the mountains, forming an inland ocean.

Figure 2.6: The Taconic volcanic islands collided with the margin of North America, forming an inland ocean. Figure by J. Houghton, after Geological Highway Map of the Northeastern Region, no. 10, 1995. 

Sediment tumbled down the mountain flanks carried by streams westward into the inland ocean, forming the Queenston Delta (Figure 2.6). Close to the highlands, conglomerates formed. Deltaic streams brought sandy, muddy sediments downstream toward the inland ocean basin to form sandstone, siltstone and shale. Settling within the inland ocean, sediments were compacted and cemented to become sedimentary rocks that stretch across to the western border of New York and Pennsylvania.

Silurian Rocks

Figure 2.7: Silurian shallow seas resulted in evaporite deposits. Figure by J. Houghton, after Geological Highway Map of the Northeastern Region, no. 10, 1995. 

The Silurian rocks exposed east to west across the middle of New York record the continuing story of the inland ocean. Sedimentary rocks were still forming with the rise and fall of sea level in the inland ocean. During the late Silurian, the ocean became extremely shallow in the Northeast. Sediments in the ocean were exposed as mudflats and rapid evaporation of the shallow seas led to the formation of evaporites (Figure 2.7). 

Figure 2.8: Silurian rocks exposed in the Inland Basin.

Much of the sediments deposited in the earlier Silurian were quickly eroded away when exposed above sea level. The rate of deposition of sediments was also slower during the Silurian because the majority of the Taconic Mountains had already been worn down by this time. As a result, relatively little sediment was preserved as rock in the Silurian, and they are therefore represented by only a thin stripe on the geologic map (Figure 2.8). 

Devonian Rocks

Figure 2.9: Devonian rocks exposed in the Inland Basin.

Figure 2.10: The Acadian Mountains in the Devonian. Figure by J. Houghton, after Geological Highway Map of the Northeastern Region, no. 10, 1995.

Devonian-aged rocks are exposed across southern New York and northern Pennsylvania (Figure 2.9). These sedimentary rocks record the Acadian mountain-building event as North America collided with Baltica. The formation of the Acadian Mountains was similar to the formation of the Taconic Mountains. Just as the Taconic mountain building during the Ordovician had formed an inland ocean and the westward spreading Queenston Delta, Acadian mountain building renewed the inland ocean by buckling the crust downward and forming a westward spreading Catskill Delta (Figure 2.10). As one would expect, the rocks of the Devonian period produced during the Acadian orogeny are similar to the rocks of the earlier Ordovician period produced during the Taconic orogeny. Conglomerates were formed close to the Acadian highlands, and finer grained sediments spread westward to form sandstone, siltstone and shale. At times, when the amount of sediment being deposited from the highlands decreased, limestone and dolostone formed as well. The Acadian highlands eroded rapidly, providing huge amounts of sediments to be deposited on the Catskill delta and into the inland ocean that were preserved as a thick sequence of Devonian-age sedimentary rocks. 

Sedimentary Structures

Upon close examination, the Devonian rocks of the Inland Basin often reveal the type of environment in which they formed by the presence of sedimentary structures within the rock. Sedimentary structures include ripple marks, cross-beds, mud cracks, and even rain drop impressions. Consider the type of environment in which you see these sedimentary structures today in the world around you. Figures by J. Houghton.

Ripple marks suggest the presence of moving water (though wind can also create ripples and even dunes). Mudcracks indicate that the sediment was wet but exposed to the air to dry and crack.

Cross-beds form as flowing water pushes sediment downstream, creating thin beds that slope gently in the direction of the current as migrating ripples. The downstream slope of the ripple may be preserved as a thin layer dipping in the direction of the current, across the natural flat-lying repose of the beds. Another migrating ripple will form another layer on top of the previous.   

Mississippian Rocks

Figure 2.11: Mississippian and Pennsylvanian landscape. Figure by J. Houghton, after Geological Highway Map of the Northeastern Region, no. 10, 1995.

During the Mississippian and Pennsylvanian period, the Inland Basin region was still an inland sea environment, with sediment being shed into the basin from the Acadian highlands in the east (Figure 2.11). Gradually, the amount of incoming sediment into the basin declined. The shoreline of the inland sea moved back and forth across the basin as sea level rose and fell during this period. The fluctuating water levels created alternating sequences of marine and non-marine sedimentary rocks, characterized by red and gray colors (Figure 2.12). Limestones were also forming in the inland sea in areas receiving very little sediment. The Northeast was still located along the equator at this time, so the warm climate created lush vegetation. Large swamps covered the shoreline areas of the inland sea. Plant material in the swamps died and accumulated as thick piles of peat. Buried by waves of sediment and more vegetation, the peat was compressed. Over time and continued burial, the peat was transformed to layers of coal. Thus, the Pennsylvanian and Mississippian rocks of the Inland Basin region, found primarily in Pennsylvania, are repeating sequences of alternating sedimentary rock and bands of coal formed because shifts in sea levels allowed lush vegetation to develop in swampy areas. 

Figure 2.12: Mississippian (dark) and Pennsylvanian (light) rocks exposed in the Inland Basin

Permian Rocks

Figure 2.13: Permian rocks exposed in the Inland Basin. 

Exposed at the surface in the Pennsylvania and Maryland Inland Basin region are Permian-age sedimentary rocks (Figure 2.13). At this time in geologic history, the continents had united to form one giant landmass known as Pangea. North America was sutured to Pangea by the collision of Africa with the east coast of North America during the Alleghanian mountain-building event, forming the Appalachian Mountains. The unification of Pangea signaled the closing of the Iapetus Ocean as well as the last time the inland sea invaded eastern North America. Though sea level fluctuated for a time, the inland ocean gradually retreated, leaving behind river sediment deposits rather than marine deposits. River sediments generally form coarser grained and more poorly sorted sedimentary rocks. With the closing of the Iapetus, the climate in the Northeast became significantly drier as the Northeast was near the center of Pangea. The lush coal swamps of the Mississippian and Pennsylvanian periods gradually disappeared as more arid conditions developed in the area. With the absence of organic-rich swampy areas, very little coal could be formed, accounting for the much smaller amounts of coal in the Permian rock record. 

Missing time in the Inland Basin

Where are the rocks representing the Triassic, Jurassic, Cretaceous and Tertiary periods in the Inland Basin? THe absence of rocks deposited during certain time periods in regions of a geologic map does not mean that there were no rocks forming during that time. It may mean, however, that very little sediment was deposited, that the sediment was eroded away, or that the rocks are buried beneath the surface. There is no single place on Earth that has a complete sequence of rocks form the Precambrian to the Quaternary. Erosion and weathering over time have removed many meters (and in some cases kilometers) of rock from the surface of the Northeast.