A highly unusual category of general coastal morphology is created by asteroid impacts at some time in the geologic past. Asteroid impact creates a circular or ovoid crater beneath which is a brecciated zone that extends many thousand meters below the former surface of the Earth’s crust. The eroded relics of these ancient craters have been called “astrob-lemes” (Dietz 1961).
Chesapeake Bay, and adjacent areas of Maryland and Virginia. The coast is characterized by an unusual pattern of a drowned dendritic drainage system, that is to say, organized like the branches of a well-shaped tree. It is fed by the valleys of the Susquehanna, Potomac, and Rappahanock rivers. The asteroid or “bolide” struck 35.2 (+/−0.3) million years ago, in Late Eocene times in soft coastal plain and shelf sediments which to the impacting object, about 3–5 km in diameter and travelling at about 80,000 km/h, would have had the consistency of warm butter. Molten glass shards (cooled as tektites) were strewn over 9 million km2. Penetrating about 600 m of youthful sediments it continued to a limited depth in the crystalline basement. The principal crater is about 85 km in diameter, and overlying (postimpact) sediments have slowly sagged into the depression and the surrounding area. In the center of this astrobleme, there is an inner depression with a peak in the center (just as in some other impact sites such as the Miocene Ries crater of Germany). A much larger area extending up to 90 km from the principal crater rim is marked by a belt of radial fractures spreading out over much of the Salisbury Embayment. A polymictous debris plume reached NE, up the Atlantic coast at least 400 km. The subsurface structures have been proven by extensive seismic profiling and drilling (Poag 1997). In the 35 Myr since the impact, the site has been the focus of differential compaction and subsidence (with eustatic revivals) that determines the general form of today’s coastline. Another but much smaller asteroid fragment at the same time created the 25-km-wide Toms Canyon crater 140 km east of Atlantic City.
Gulf of Mexico (Chicxulub crater). This impact occurred near the west side of the Yucatan peninsula of Mexico about 65.2 (+/−0.4) million years ago, generating a crater 180 km across. Its ejected debris has been traced to a semicircular rim extending in an arc through central Texas which suggests an incoming orbit heading NNW. This event was destined to become the Cretaceous/Tertiary boundary, marking a revolution involving major biotic extinctions, notably the dinosaurs, ammonites, and other classes (75% of all marine species). While the western and northern borders of the Gulf of Mexico had already developed as a broad arc in Mesozoic times, its southern border is marked by a meridional fault system that today truncates the western borders of Yucatan, a nearly horizontal platform of Miocene limestones. The impact theory, initiated by W. and L. Alvarez in the late 1970s (from evidence in Italy) triggered an intense interest that has been well reviewed by Marvin (1990) and expanded to events throughout the entire Phanerozoic (Rampino and Haggerty 1996). It seems likely that anomalous patterns of coastlines worldwide should be explored for possible astrobleme ancestry.
Hudson Bay. The present form of this embayment is an epicontinental sea bounded particularly in the southeast by an extraordinary arcuate coast that coincides with an Archaean/ Proterozoic boundary injected by lopolithic sills of diabase. An ancient astrobleme is suggested by Dietz (1961).